I have been having the same issues all day, my sim card is not 5G ready, have had it a few years now and speeds of around 44/47mbps using my S9. My Ultra came today and when I first put the card in my data speed went down to 2/3mbps after a lot of playing about with it trying everything I could think of its now getting to around 6/8mbps. When I swap the sim back into the S9 its back up to mid 40s. This is really disappointing this phone is a real let down if I'm honest, will try going to my network provider to see if a new updated 4G card works any better if not then this phone will definitely be going back to Samsung.
well mine turned on that i was not in a verzion area so mine was hooked to a tower a state near by.
but once i got home my speeds did get better.
still do not understand what this truely means but things got better so i do not mind.
----
did you find any thing that worked for you to fix your problem by chance?