Thank you for the tag @ScruffR
I’ll be more than happy to help you get your setup going. You are right though. These are not toys that you just plug in and they just work on their own. A product capable of plug and play connectivity like that would set you back several hundred dollars like a PLC would. These products are meant to be a cost effective hardware solution for developers. It is completely understandable how you might be frustrated using products like this if you don’t have experience with code development.
I would say the first step is going to be getting all of your hardware connected up properly.
As ScruffR mentioned D0 and D1 are used for I2C communication which is required for communicating with the Current Loop receiver boards you have so you cannot use those for your digital inputs, but as he also mentioned you have A0-A5 on the other side of the module which can be used as digital inputs(you could of course also use the DAC,WKP,RX,and TX pins as well). That should give you more than enough digital I/Os.
On the PPEScrew install the D0/D1 jumpers, this connects Particle’s D0 and D1 lines to the I2C connector at the end of the board. It also pulls the lines high for I2C communication which means you do not need to install the pull up jumpers on the Current Loop Receiver boards.
Each of the Current Loop Receiver boards have two sets of address jumpers, this is because each board has 2 I2C chips(MCP3428). The MCP3428 has a default starting address of 0x68. The address jumpers can be used to change that address. You need to use the jumpers to set each of the 4 chips on the two boards to unique addresses. On the first board leave the jumpers on the first set of address jumpers off, install the 0 Lo jumper on the second set of address jumpers. On the second board install the 0 Hi jumper on the first set of address jumpers and the 1 Lo jumper on the second set of address jumpers. This means you will have 4 I2C bus addresses which will be 0x68, 0x69, 0x6A, and 0x6B. Then you can individually interact with all 16 channels of 4-20mA inputs in your code.
Once you do all of this your hardware is ready and you can begin developing the code or hire the code development out if you do not have time to learn from the ground up.