Hi Contacting OPTUS is almost impossible and very very tedious. My problem is we recently connected to the nbn via a new optus service. Sort of works ok. We chose the $80 with fetch mighty plan. I have a fetch mini from their mobile service and it works fine, but of course cannot record. The fetch mighty box works OK except will not pick up fta stations. Please note that it is NOT the fault of the antenna, we are able to watch all stations connected direct and were able to use the mini. We just get pixelated pictures and cannot get any ABC or SBS channels. Trying to call optus didn't work as they won't answer and even the long winded "live chat" says try later. This is a tuner issue but how to get the box changed??
Its possibly (likely) still an antenna issue. The nature of digital TV is a small drop in signal strength can result in some equipment still get 100% of the image and others 0%. Its called the digital cliff. The Fetch PVR is known for having slightly weaker tuners than other equipment (so a change over is likely to not fix the issue.)
You can contact Optus livechat (24/7) see if you can get the Fetch department but its not really needed. Just request a new box be sent out. They will also send out a satchel to return your current one in. Note the moment you make the request your existing box will likely be remotely deactivated.
Have you tried tuning the Fetch PVR with only the antenna lead going into it? i.e. no splitters to TVs or other equipment.
IMO a more likely solution would be to boost the signal to the Fetch Box. Something like this is a quality brand and should get the signal back up over the edge of the cliff. Find them online or in electonics stores.