I have had this problem for months and cannot find a resolution. Some apps will not load data from the internet over wifi, but are fine as soon as I stop wifi. All the time things like Chrome are working with no problems. Examples:
Weather - will not refresh
BBC Sounds - will not show content or My Sounds
Flight Radar 24 - shows maps, but does not show aircraft
In each case I just get the spinning wheel. These things all burst in to full life as soon as I drop the wifi. Also seem to work for an hour or so after a big network disruption (reboot router, drop and reconnect to wifi). I have the same issues on my current S20 and also on my old S9 - even after doing a factory reset and only installing the problem apps.
BT (my ISP) are sending a replacement Smart Hub 2, but I'm not convinced it is the router.