The Mobile Hotspot on my S20 5G was broken after the April 2021 update. Samsung asked me to send it back, and when it came back, it still didn't work even though they said that there was no problem found. I'm now on my 3rd S20 5G, and while it was back with Samsung the first time, the data went out on my old trusty S7 while Samsung had my S20 5G.
As it turned out, the SIM in my old trusty S7 went bad which is why my S7 couldn't receive data. I received my S20 5G back from Samsung, and 2 days later, I got the new SIM card from my carrier. I installed the new SIM, turned it on, loaded my apps and settings, and then it installed the August 2021 and September 2021 software updates.
I now have a fully functional S20 5G with a Mobile Hotspot. The problem is that I can't say for sure if the September update fixed the problem or if my SIM was bad in April but would still function in my S7 but not in the the S20 5G.
If you're having problems with the Mobile Hotspot in your S20 5G, download and install the September 2021 software update. If it still doesn't work, get a new SIM. Problem solved!...Sort of...