Sounds external yes

Best of luck!

Looks like you have the bell wire connected as well. Not required with filters or filtered faceplate and can cause interference.

No harm in double filtering as connected Sky boxes needed double filtering.

Best to use Cat 5e cable for extensions and not telephone wire/bell wire as the twisted pairs reduce interference.

If you can test the system with the modem/router plugged in to the master socket (with a power supply via extension lead for the modem /router) and an Ethernet cable up stairs.
Not ideal, I know, but you have to rule out any issues with your own wiring.