I will install my outdoor omnidirectional antenna hopefully within the next few days.
In the meantime, I installed an App on my Android phone called "LTE DIscovery", and so can confirm my phone (and thus presumably my 4G router) is now using LTE Band 20, i.e. the lower capacity 800MHz signal.
Out of interest - are there any 4G routers where you can select which LTE Band(s) to connect to? That would help solve the problem. For example, the Huawei B525
looks interesting, and there are other similar 4G routers, but I can't find any further detailed technical info. It seems that this feature is available for some USB 4G modems - for example, see this YouTube Video
- a 4G router with an option to lock onto a specific LTE band would be perfect!
In the meantime, luckily, it does look as though a local FWA (Fixed Wireless Access) scheme for my village *may* well go ahead within the next few months (although no guarantees), which should provide hopefully a nice stable 30Mbps downstream, if it does.
Thanks again everyone for all your information & ideas.