So this is my 2nd s10 plus. I sent my first one back for this same issue and this issue still arises with the new s10.
The WiFi connects to the virgin router fine. But randomly when browsing the Web, it will slow down and struggle to load pages, and only upload (the arrows next to the WiFi signal) and the 4g sign will appear whilst the WiFi sign is still showing. I have turned switch to mobile data off also.
I have an ipad and an s8 also so when this problem arises I have another device to see if the same issue occurs, however these devices are working fine.
This happens multiple times a day and can get very annoying.
Could it be that I'm unlucky and both phones have WiFi issues, or is this an issue with the s10 plus it self.
I did think it is the router but other devices connected to the same router works fine
Any help is much appreciated
Id check your router settings. Since mine was Ubiquti AC-AR model. All devices:mothers iphone, fathers Huawei and tablet were ok.
Except my brothers and mine samsung s9 and s10.
I disabled the features in router that "optimise the network" or in any way meddle with which device connects to what radio 2.4. Or 5.0.
That instantly fixed the same problems you described.
Sorry if Im not helping.