I’ve been having this problem throughout the iOS 13 beta as well as the beta for iOS14 and now the released versions/update. I have I iPhone XR easy solution. leave all cellular setting as default and turn OFF the VoLTE setting. roaming and other setting changes such as “only 3G” didn’t do much for me. Sure the signal seems strong but internet on cellular slowed to a crawl and pages such as safari stopped loading. hope this helps. feedback appreciated. Thx