I think the cable runs to that connector
through the wall from outside, but yes, I should have a better look to see if it's running down the outside wall from an overhead cable. It then runs along (clipped) to the inside wall - I think it feeds the NTE socket, but again, I need to spend a bit...
We're having a section of external wall, currently plywood I think, rebuilt with block, cavity and brick.
The phone connector (see pic) will end up in the cavity, so we're thinking we need to build an access box into the inner (block) wall in case any work is ever needed on this connector.
How...
I found this really helpful in making sense of damp walls:
https://www.worldofbooks.com/en-gb/products/damp-house-book-jonathan-hetreed-9781861269669#GOR004981750
To some extent, successive governments have installed them, eg; through grants and FIT.
Apple trees are good, but the government aren't installing them in my garden...
Sorry but I can't help thinking that grout sealant, bathroom ventilation, possibly retiling or bathroom underfloor heating are going to be the kind of solutions that will work for you...