iOS 13 had an issue where it was working with WIFI frequency of 5Ghz only ; therefore, the fix was to disable 2.4Ghz on the WiFi router. (For AT&T users) Go to router settings http://192.168.1.254/cgi-bin/home.ha and login with your credentials; go to Broadband; then under that go to wifi; you may need your router access code printed on the router; then advanced options; click disable on the 2.4Ghz; scroll to the bottom and click save.
Fix 4: Update to latest iOS
The permanent fix for the issue should be released by Apple in upcoming releases of iOS by Apple. So it is recommend to always update iOS to latest version. Go to setting -> about and check for any software updates pending for you.