Thank you both for your responses.
I've tried adding 213.3.0.0/17, but to no change sadly.
I have noticed today a strange (at least to me) behaviour where it is recording TV and playback of the recorded content is fine right up to the point where it would be live, but Live TV displays "No TV Signal is currently available from this channel" on multiple different channels.
Tested as a workaround going over WIFI provided via the Swisscom Internet Box therefore bypassing our Unifi equipment. But on screen instructions ask for the + Button to be pressed on the Internet Box which the TV Box does not detect.
Distance between both devices is about 4 meters which shouldn't be a problem, and other mobile devices such as a laptop and tablet can detect and connect from further.
Wired directly from Swisscom Internet Box to TV Box isn't really possible at the moment, as I have no ethernet cable long enough to cross the distance. (wife wouldn't like a flying cable going across our living room I bet also haha)
This has me quite puzzled.