Hi Stratoshere, thank you for posting. Sorry to hear you of your recent speed issues on 4G.
Have you done a reboot on your handset? Is the problem persisting following the reboot?
If you can also confirm:
-Make and model of Handset
-Software currently on Handset, and if there are any software updates required (if this is the case please install these!)
-If there is coverage showing in your area.
To check your coverage please go to this page. About halfway down there is a button for 'Can I get 5G?' This will allow you to look at coverage in your area, using your postcode in the search bar.
Let us know how you get on so we can offer further support! All the best.
Molly