Moved house and having a lot of trouble getting ADSL or Fibre set up.

After a month of no internet decided to get a new sim for my old iPhone 4s which has been set up as a hot spot for the last 2 months with computers and phones connecting direct to it (unfortunately our normal network has no service here either)

Its now going to be a couple more months before any proper solution comes about and so would like to get a slightly better set up, if possible. Is there any way of connecting my Time Capsule and iPhone together so that the items wired to my TC can use the 3G of the iPhone?

I did see some people from a few years ago talk about using the "join network" option
of network type but my TC only has create or extend network and having tried extend it doesnt seem to work.