Thank you very much for your reply @rickkas7
I have made some progress and it works but not always 
I have used Curl to send the GET request to my server and monitored the traffic with Wireshark. So I found out how the request needs to be formatted:
Curl request:
curl -GET “http://mydyndnsaddress:8086/query” --data-urlencode “db=sensors” --data-urlencode “q=SELECT LAST(kWh) FROM sensors WHERE Location=‘street’”
curl -GET “http://mydyndnsaddress:8086/query” --data-urlencode “db=mydb” --data-urlencode “q=SELECT LAST(value) FROM cpu_load_short WHERE region=‘us-west’”
Wireshark:
Frame 15: 227 bytes on wire (1816 bits), 227 bytes captured (1816 bits) on interface 0
Ethernet II, Src: IntelCor_ae:db:c8 (00:24:d7:ae:db:c8), Dst: Elitegro_77:e4:7a (b8:ae:ed:77:e4:7a)
Internet Protocol Version 4, Src: 10.0.0.17, Dst: 10.0.0.14
Transmission Control Protocol, Src Port: 1532 (1532), Dst Port: 8086 (8086), Seq: 1, Ack: 1, Len: 173
Hypertext Transfer Protocol
GET /query?db=sensors&q=SELECT%20LAST%28kWh%29%20FROM%20sensors%20WHERE%20Location%3D%27street%27 HTTP/1.1\r\n
[Expert Info (Chat/Sequence): GET /query?db=sensors&q=SELECT%20LAST%28kWh%29%20FROM%20sensors%20WHERE%20Location%3D%27street%27 HTTP/1.1\r\n]
Request Method: GET
Request URI: /query?db=sensors&q=SELECT%20LAST%28kWh%29%20FROM%20sensors%20WHERE%20Location%3D%27street%27
Request Version: HTTP/1.1
Host: 10.0.0.14:8086\r\n
User-Agent: curl/7.49.0\r\n
Accept: */*\r\n
\r\n
[Full request URI: http://10.0.0.14:8086/query?db=sensors&q=SELECT%20LAST%28kWh%29%20FROM%20sensors%20WHERE%20Location%3D%27street%27]
[HTTP request 1/1]
[Response in frame: 17]
Server response:
{"results":[{"series":[{"name":"sensors","columns":["time","last"],"values":[["1970-01-01T00:00:00Z",0.13]]}]}]}
I have also modified the code to read the incomming chars and wrote a complete new sketch just for reading the database. This sketch runs on a Particle Core:
#define INFLUXDB_HOST "my dyn dns address"
#define INFLUXDB_PORT 8086
#define INFLUXDB_DB "sensors"
#define DEBUG_SERIAL true
#define SKIP_CHARS 0
#define BUFFER_SIZE_MAX 1024
TCPClient client;
unsigned long previousMillis = 0;
unsigned long previousMillis2 = 0;
unsigned long interval = 15000;
char buffer[BUFFER_SIZE_MAX];
void readInflux()
{
int i = 0;
int j = 0;
int k = 0;
bool printOnce = false;
int i = 0;
int j = 0;
int k = 0;
bool printOnce = false;
if (client.connect(INFLUXDB_HOST, INFLUXDB_PORT)) {
client.print("GET ");
client.print("/query?db=sensors&q=SELECT%20LAST%28kWh%29%20FROM%20sensors%20WHERE%20Location%3D%27Bachgasse%27");
client.println(" HTTP/1.1");
client.print("HOST: ");
client.println(INFLUXDB_HOST);
client.println("Connection: close");
client.println();
client.flush();
} else {
if(DEBUG_SERIAL) Serial.println("\r\n\r\nConnection Failed!");
client.flush();
client.stop();
return;
}
// wait 5 seconds or less for the host to respond
uint32_t startTime = millis();
while(!client.available() && (millis() - startTime) < 5000);
if(DEBUG_SERIAL) Serial.println("\r\n\r\nREADING HOST DATA......");
uint32_t lastRead = millis();
// If the host doesn't close it's connection, we'll timeout in 10 seconds.
while (client.connected() && (millis() - lastRead) < 10000) {
while (client.available()) {
char c = client.read();
if(DEBUG_SERIAL) {
Serial.print(c);
if(i++ > 100) {
delay(5);
i = 0;
}
}
if(c == -1) {
Serial.print("\r\n\r\nERROR......\r\n\r\n");
client.flush();
client.stop();
}
if(j++ >= SKIP_CHARS) { // don't buffer the first X bytes to save memory
if(DEBUG_SERIAL && !printOnce) {
Serial.print("\r\n\r\nSAVING......\r\n\r\n");
printOnce = true;
}
buffer[k++] = c; // save character to buffer
Serial.print(c);
delayMicroseconds(100);
if(k >= BUFFER_SIZE_MAX) { // if we reach the end of our buffer, just bail.
Serial.print("\r\n\r\nOUT OF BUFFER SPACE......\r\n\r\n");
client.flush();
client.stop();
}
}
// as long as we're reading data, reset the lastRead time.
lastRead = millis();
} // END while(client.available())
} // END while(client.connected())
client.flush();
client.stop();
if(DEBUG_SERIAL) {
Serial.print("\r\nCHARACTERS RECEIVED: ");
Serial.println(SKIP_CHARS + k);
}
}
void setup() {
Serial.begin(19200);
//kwh = readInflux();
}
void loop(){
unsigned long currentMillis = millis();
if(currentMillis - previousMillis >= interval) {
Serial.println("reading Influx");
readInflux();
previousMillis = currentMillis;
}else{
Serial.println("Device not ready");
delay(1000);
}
}
Serial output:
Device not ready
Device not ready
Device not ready
Device not ready
Device not ready
Device not ready
Device not ready
Device not ready
Device not ready
reading Influx
READING HOST DATA......
H
SAVING......
HTTTTPP//11..11 220000 OOKK
CCoonntteenntt--TTyyppee:: aapppplliiccaattiioonn//jjssoonn
RReeqquueesstt--IIdd:: bb55ccbb55557777--2255bbff--1111ee66--9933aa55--000000000000000000000000
XX--IInnfflluuxxddbb--VVeerrssiioonn:: 00..1122..11
DDaattee:: SSuunn,, 2299 MMaayy 22001166 1177::0066::4400 GGMMTT
CCoonntteenntt--LLeennggtthh:: 111122
CCoonnnneeccttiioonn:: cclloossee
{{""rreessuullttss""::[[{{""sseerriieess""::[[{{""nnaammee""::""sseennssoorrss"",,""ccoolluummnnss""::[[""ttiimmee"",,""llaasstt""]],,""vvaalluueess""::[[[[""11997700--0011--0011TT0000::0000::0000ZZ"",,00..1166]]]]}}]]}}]]}}
CHARACTERS RECEIVED: 318
Device not ready
Device not ready
Device not ready
Device not ready
Serial output when it is not working:
Device not ready
Device not ready
Device not ready
Device not ready
Device not ready
Device not ready
reading Influx
READING HOST DATA......
CHARACTERS RECEIVED: 0
Device not ready
Device not ready
Device not ready
Device not ready
Device not ready
The code I am using to read the characters is from @technobly: https://github.com/technobly/SparkCore-FacebookLikesAlert/blob/master/application.cpp
It seems to read the characters so fast that it is reading them multiple times. Is there a way to handle this?
Edit: I have not changed anything but it stopped working: CHARACTERS RECEIVED: 0
Edit2: I have found the Error 
Now I only need to solve the problem of multiple characters.
Edit3: I have forgotten to set DEBUG_SERIAL false 