Just wanted to post as I found a solution by using the threads above and some info from another site. I have a new Three 5G ZTE Hub in good range of the tower.
I had a PS5, and Samsung mobile not getting good upload speeds (downloads fine). PS5 was 8.8kbps (yes kbps not mbps), and the Samsung phone zero. Also an iPad and iPhone getting great downloads and uploads. Called Three support who were useless, cut me off once, passed me from broadband to mobile and back to broadband, insisting their network was fine as the iPad and iPhone worked fine and therefore to call Samsung. Lo and behold they were wrong, reading from a script and desperate to get me off the line!
Decided to try and puzzle it out and resolved it by changing the APN settings in the router (type into browser:
http://192.168.0.1/index.html#login). I needed to change the APN from 3internet to three.co.uk
Turn off the switch below, then click Settings underneath:
View attachment 2320
Then select APN settings in the next screen (no image supplied of this), then you’ll see the screen below, change the mode to Manual, Add New, then use the values below. Once done go back to start page, switch the 5G toggle back on and then do a speed test to check its fixed.
View attachment 2317