Thanks for the replies.
I have since ported my number over and am only using the one sim now. I have also deleted apns, re-added apns, added apns with only internet, apns with both internet and mms centers, etc. Restarted phone after adding, etc, etc. It still happens. Notably longer before it happens sometimes, maybe 5+ mins.
Just to be clear this is only happening when I am downloading at high speeds (1Mb/sec+). The download speed drops lower and lower and then disconnect in top right of screen from 4g to nothing. 3 seconds later 4g re-appears and download picks up speed again. Sometimes it just disconnects immediately at high speeds for 3 seconds but there is often a noticable drop in speed a few seconds before it disconnects.
When I am not downloading this doesn't appear to be happening. (Need to keep an eye on it more but I don't think it is)
Like I said this literally NEVER happened when using the same network and mast for the last 5+ years with three and 1+ years on this exact phone. I am far from an expert on this but I must say to me the stability of this phone and previous network still make me believe it is not the phone, however, like is said, I'm no expert 🙁