@DottyD: Next time the issue occurs, please unplug your router at the wall for 30 seconds, then plug it back in and allow time for the reset process to complete. If your Galaxy S21 reconnects following this step, then the issue is likely to be stemming from your router.
If the problem persists, please can you confirm if you see an exclamation mark (!) next to your Wi-Fi symbol in the navigation bar when the signal drops? If not, it may be worth heading to Settings > Connections > Wi-Fi > On > Tap the cog next to your router > Auto-reconnect (toggle this on) > Back > Tap the 3 dots in the top right > Advanced > Turn on Wi-Fi automatically > Off. By disabling this feature, it will eliminate the possibility of any automated disconnects based on your current location.