I know there are a lot of posts on this topic but in searching them they are all quite old and may no longer be valid comments for 2020 so I am starting afresh.
I am constantly receiving the message there are streaming issues when in catch up TV. Never a problem with any of the "live" channel whether FTA or subscription. No problem on laptop and a speed test always results in around 44mbps download and 18mbps upload give or take 2mbps.
It is just the catchup TV.
I have tried the troubleshooting - resetting the Fetch box, resetting the modem, etc. Nothing works.
We are on NBN via HCF. The copper cable element is all new as our initial NBN roll out was delayed 18 months when it was found the existing Telstra cable was too corroded to work (don't ask why that wasn't an opportunity to change to all fibre).
A few ideas I would try.,
1) Disconnect the ethernet cable and connect by wifi to ensure there isn't a problem with the ethernet connection.
2) Perform a hard reset of the Fetch box. This will remove any recordings stored on the box. Ensure you have the activation code to hand for when you restart the Fetch box.
3) Perhaps not so easy with social distancing. If you have a family member or friend with a Fetch box see if you can borrow it for a few hours and set it up at your home. Alternatively take you Fetch box around to a family member or friend and set it up in their home. This should be the ultimate proof whether it is your Fetch box or your internet connection.
If it does seem to be the box contact Optus and ask for a replacement Fetch box.
I have done all of your suggestions.
That said, I think I have narrowed the problem down to being specifically a problem with the UKTV channel.
I can watch TV all day FTA or pay channels. I can watch FTA catch-up channels with no problems. Even the pay TV catch-up channels I watched yesterday - Universal, Oxygen BBC First - had no issues.
I then started to watch a show on UKTV and within 20 minutes it froze with a streaming issue message. So, this time, I pressed the pause button and went to make a coffee, check the mail box (outside) and basically spent about 4 minutes away. Pressed play and the show continued for its entirety without another issue.
I have now come to the conclusion the UKTV channel may have a buffering problem. I will experiment more this afternoon by doing the same as above and seeing what happens.