Everything would stop working if you had two powered-up routers connected. If the Sky didn't come back with turning off the Vodafone one, that is surprising. But I gather your prime purpose was to have it all set up ready for Vodafone.
I hope the upstairs phone is on a filter! A Sky one would be fine.
Longer term I still advise checking to find the master and remove the bell wire on T3 at it.
There is a possibility that there is a junction box on the incoming line before the master, feeding both the known sockets separately, rather than the master and the master feeding the extension. That could be really bad news. It would be what is called a "bridge tap". Again, the FTTC will still work, but there could be a lot of errors that might cause DLM to really clobber the connection speed.
Does your Dad normally turn his router off at bedtime? Normal practice is to leave it on, but again it's not a disaster if he does.
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