I have tried 3x  5G routers (2x used, 1x new).

The first one was a used Vodafone gigacube, which I had been told was unlocked. It failed to connect at all to either the ID network, or the Vodafone network (using a talk mobile SIM). I was refunded, happily. 

The second one was a new, cheap Chinese unbranded model, BZ50N1.It connected to the ID network, but I was getting 30%+ packet loss and it was unusable. Again, I was refunded.

My 3rd one is a used BZ50N1. It connects, and works perfectly on 4G, but I get 5-10% packet loss on 5G.

At this stage, I am unable to identify if the issue is with the router or the mobile network. The 5G signal is strong (3 out of 4 bars). I ran traceroutes on the 5G and 4G networks. The 4G data is routed through Three's network and works fine. I suspect (but am not sure) that the 5G data is routed via Vodafone, none of the mobile network routers responded to the Traceroute ICMP packets (only 2 did on the 4G Three network). On both networks, it looks like they exit the mobile network into a GTT backbone network in Germany.

I will take my router ti a different location in a few days to see if it is an issue with my local tower. I will also try it with the Talkmobile SIM if thhe problem is still evident at the other location.

Can anyone suggest something else I can try to establish whete the problem lies?