I recently tested Boron LTE in Sweden with 3rd-party SIM from Telenor and it works.
When I started it just flashed green all the time and every time I tried to set MVNO to 100 ( Europe) I get an error.
Then I noticed that firmware on onboard SARA R410 modem was to old ( from 2019) and understood modem firmware was the problem.
So I find this thread https://community.particle.io/t/ublox-modem-firmware-update/50205 but instead of using proposed programmer I soldered an USB cable to modem test pads on PCB.
Then I asked Ublox for newer firmware. After some discussion Ublox send me latest firmware, so I upgraded firmware and then I was able to set MVNO to 100.
See also https://forum.arduino.cc/t/firmware-upgrade-for-ublox-sara-r410m-02b-on-the-mkr-nb-1500-2/699292