That's fair enough, as you say, it is lacking somewhat on the WiFi side.
Looking up your cell ID, that mast (which is located just off the A6006 here :
https://goo.gl/maps/E4i2Mh7dTKv2ksiU6) does seem to have only Band 3 for Three from the data that has been collated by cellmapper. There is a
chance that there are more but no-one has collected/submitted data to cellmapper for it yet.
Given that, if you're intent is to stick with Three, then any 4G router that satisfies your needs on the WiFi side would suffice on the 4G side (99%+ support Band 3).
If you wanted to future proof a little, getting one that can aggregate Band 3 and Band 1 (and/or Band 28, though this is
fairly new/rare) would be a good step up, for when those frequencies arrive, though how long that might be no-one knows (could be tomorrow, could be in 5 years!).
Going further you could consider a 5G unit but those are much more expensive and there is still no telling when 5G might come to your mast.