@finbarro The only box you can currently purchase is the Swisscom TV Box 21 (see the list), all the others are no more pruchased. I suppose you have the TV Box (UHD), possibly you can call the Hotline 0800 800800, to explain wht’s going on, and they can deliver you a replacement.If you still have the TV Box (HD), then you can change it for free, as it reach their end of lifecycle and were replaced.
blue TV: devices & instructions - Help | Swisscom
So you wrote, you tried all the suggested fixes? So, you made already a reset, with the paperclip in the tiny hole in the box? Did try already a newer HDMI cable?