Simple answer: it is 5G.
Correct answer:
'G' means generation, it says nothing about technologies. It is misleading terminology created by marketing people. We were hooked on this terminology something like 7 years ago too and it was a mistake to add it to the name of the product iSocket 3G, which is now (since 2021) obsolete for USA.
Technologies we use now in the new iSocket called eMTC and NB-IoT - see Wikipedia.
These are latest technologies created specifically for devices like iSocket. You don't actually need to think it terms of "G", you just need to know that with these technologies your iSocket will work for 10+ years (likely 20+). These are latest technologies which are not even deployed in some countries, however USA is fully covered with eMTC. Again, the new iSocket represents a fully new approach - you don't think about tech, networks, coverage in your area, SIMs, carriers, contracts, difficult settings, etc - you just receive a Plug-and-Go solution that will work on any location and will last.
If you like to speak in marketing terms of 'G' we can give a simple answer: it is 5G (see the same link to Wikipedia).