i have the same issue with S21 ultra wifi, it is ok during speedtest, ok to play youtube etc. however at the same time to download/update apps, it takes forever and mostly fail ie to install galaxy buds pro is only 11mb but it could not be completed. now i cannot restore my whatsapp due to this also. i tried the same efforts in my old S9+ without issues at all.
speed test showing 18Mbps
Same here, got my S21 Ultra now for 5 Days .Open a game or my Bank would load a very long time or cancle. Only on Wifi. ON Mobile Data there is no problem. But Speedtest give me full power. Cant find the problem.
I had the same problem. When joining some networks I get connected and after 1 second it says "Connected without internet" or it couldn't find an IP-adress. I found a fix on another forum for the S20 and this worked for me:
Turn off change to mobile data automaticly
Set your MAC adress to phone mac.
And there you have it. I also turned off Google Location Accuracy, haven't tried to turn it on after. It seems like this is a Samsung issue that isn't fixed over several models.
Link to forum that helped me: https://us.community.samsung.com/t5/Galaxy-S20/Samsung-s20-ultra-dropping-WiFi-connection-constantly...
Second to last comment on page 6, and I also turned on phone mac adress. One hour now and fully connected to the office network after no connection yesterday and this morning.
Hope this helps and a fix from Samsung soon!
EDIT: Apps on wifi are still really sluggish while speedtest says full download speed. It might be that most apps aren't updated to support this phone yet or full support of Android 11/One UI 3.1. It seems that they are way slower to load when I haven't used them in a while, but this is my second day with the phone so I guess the AI hasn't got a hang of my usage yet and that might be the real problem here.
In the WiFi settings you go to the settings for the network you're connected to (the * on the right). Furthest down you go to advanced. Press the MAC-adress type and choose phone MAC (I have my phone on norwegian so I hope I've translated it close to english).
I still have some apps that sometimes work on the office network and sometimes don't, so this is not the solution for speedy surfing around. At home I have absolutely no problems with the speed, so I guess the phone hates old routers...