I have a mobile data connection problem, and not only I but friends who own a Galaxy 10 as well, and it is very, very annoying. The moment I leave my home, or other place where I am connected to WiFi, and the WiFi connection is lost, Android 10 (or the Galaxy 10+) is unable to connect to mobile data. All apps are losing their connectivity and the up-arrow next to the connection bars is lightened all the time.
The only way is to a) reboot my phone (works only temporay, have to reboot multiple times as the problems returns) or play with toggle on/off/on/off Airplane mode. I would have hoped that Samsung would have a fix by now. BTW is started to happen after my Android 10 upgrade. I am driven crazy by this "feature" almost to the fact that I will buy another brand and eave Samsung all togther
Under your Wi-Fi advanced settings, have you verified that Wi-Fi Power Saving is turned off, Automatically connect to Wi-Fi is turned on and I believe there should also be an option to switch between Wi-Fi and Mobile Data automatically.