This is on an AT&T S20 FE, but it should be similar. At least my S9 is similar.
"Settings" > "Connections" > "Data usage" > "Mobile data usage" > there should be a list of apps below the usage graph (which shows all mobile data from every app), scroll down past the ones that are "restricted" if any, and find "Mobile Hotspot and Tethering". That should show how much the phone says you used. The gear at the top lets you set the billing cycle. Mine resets on the 4th of every month.