This thread, on Vodafone Ireland's support forum, suggests that a fault in the service provider's data settings can result in the loss of 4G on an iPhone. (The questioner's problem was fixed by the intervention of Vodafone's support staff, so it seems likely that your own service provider can get that issue sorted out for you too if you contact them):
https://community.vodafone.ie/t5/iPhone-Apple-Devices/No-4G-suddenly-anymore-on-iPhone-7-plus/td-p/207478
With regard to your phone and your router not wanting to talk to each other, have you tried turning off the router, waiting a while and then powering it up again? (Yes, I appreciate that other devices can connect without any problems, which might suggest that the fault lies with your phone and not the router, but reading through posts here for many years suggests that, however illogical it might seem, resetting your router might actually be what's needed here).