I gave in and bought a Huawei H122-373 as Three claimed 5G is now available in my area and I should get indoor coverage.
Unfortunately it seems my PAYG Three SIM cards have expired but I was able to do a network scan and its only showing 4G for all networks anyway.
On the up side, I just got the fastest speed test on Voxi that I've ever had at 150/30 which with all connections combined resulted in my best speedtest.net result ever of 249.43/55.
My previous record had been 233.51/52, so a very expensive minor upgrade.
On the up side I will at least be able to keep a lookout for 5G in the area and this model DOES show bridge mode, the B535 did not.
If we weren't still in pandemic land I could go across the road to try higher ground, might be able to get a signal there although being at the edge of 5G coverage vs right next to the 4G mast, it probably would be slower.
UPDATEJust tried with my phone connected to the H122 WiFi network before going to bed and beat my previous record on Voxi.