Settings > General Management > Reset > Reset Network Connection.
Clear Cache Files from the phone's System Partition Section > https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ You may need to insert your earphones or hook upto a powered On Laptop to access the Clear Partition Section. This may help with lag or stutters as this clears the phone of old/unused or broken cache files.There is also a Repair Apps option in the Menu System too.
Can you try your sim card in another phone and if possible another sim card in your phone as a process of elimination to rule out the sim card or phone as the potential issue.
Run the diagnostic tool in your Samsung Members app to see if anything shows.
As a last resort back up and Factory Reset.
A Samsung Service Centre can take a look at your phone.
If I can be of any further help then please don't hesitate. Take care. 😎
