We can have 5 bars 4G LTE yet no data will be shared. Can't load anything from a simple clip from YouTube to satellite radio not working. My Hotspot works just find during this time.
What carrier are you using? I noticed AT&T had a recent issue similar to this. It appears to impact texts as well. On my end I think advance messaging or smart wifi maybe the cause. When I opened the hidden "developer options" on my phone I turned on a setting that keeps your mobile data active even when on wifi. That drastically improved errors or no responses. Messaging is still a little problematic for me.
I made a call in my area and they were not aware of issues. They updated a packet that reconnects service to local tower. I went from amazing 5g in my living area to a horrible opposite. I do notice it fluctuates. Areas previously with good 5g are dead zones now and areas not centrally near 5g towers seems to improve the matter. I'm noticing that along with poor reception I'm not getting advance messaging "read & delivered" receipts. I'm almost willing to bet the updates AT&T is doing internally with their website and messaging services is a cause. Most carriers have dedicated all samsung phones to transition from Samsung messages to Google messages. S21 came factory default this way except for the US market. Some reason US was excluded. Now att is changing directions on that. Migrating services always has hiccups. Your frustrations are shared and hope for a quick resolution.