I too have a mifi (HUAWEI) router and I’ve not had a problem with it which is no help to you I realise. If it were me I’d take out the sim and try it in a compatible phone to see if the coverage differs in which case it would point to an issue with your router. You could always check both the Three status page to check for tech issues in the area you’re currently in and the Three coverage page. To say (if I’m understanding you correctly) you only get reasonable coverage in one area sounds strange to say the least. Finally and if all else fails you could call tech support on 333 option 4 option 3 and option 3 again.
