5G router connects to internet but won't provide internet - force to 4G?

I foolishly bought a 5G router from China - the specs are great and it seems to work, only it doesn't like my T-mobile SIM. I bought an unlimited weekly 5G sim from HT.HR (part of T-mobile). When inserted into the router it connects to the network on 5G

However my PC connected (via wifi or ethernet) will not get internet through this router.

If I put in another SIM from a different mobile network, it does get internet OK on the PC.

There are v few settings on this router but I’ve put in the APN, and every setting I can re. PAP/CHAP but nothing helps, and ht.hr and the router supplier are no help.

Oh if I put this sim into a 4G mobile phone, it will connect and give me internet as a hotspot.

My idea was to try to force the SIM/Router to use 4G instead of 5G, but no idea if I can or how to do it. ¿Tienes alguna idea?