MAX BR1 MK2 - SIM cards that work?

Hi all - we have a MAX BR1 MK2 on a boat. We had a Google FI sim in it in 2019, sailed from the US to the Caribbean where it worked well, and then during 2020 they cut us off for being out of the country too long. Now we’ve just returned. After some hassle, they turned the data back on again, which works fine with the voice/data sim in an Android, but the data-only sim in the MAX doesn’t work - connects, but gets like 200kbps only while the phone next to it is getting 100mbps. Fi says it’s not their problem (of course). We bought an ATT prepaid 100GB, doesn’t work in the MAX at all, though works fine in an iPad. So the question is - what sim are people using in their MAX routers???

I see a similar post found at AT&t SIM issues with 5G - #41 by dhw2