Without any doubt Samsung Galaxy S10 Plus far superior phone than Oneplus 7, basically there is no comparison. S10+ wins the race in any aspect, even though Oneplus 7 has SD855 but it's too laggy phone. I won't suggest anyone to buy Oneplus 7 at all. Samsung flagship phones have brilliant display once you are addictited to it you wont like any other phone screen.
Kindly follow the mentioned steps to resolve the issue in your device.
1) Check Network mode and set it to Auto Connect. (Settings > Connections> Mobile Network> Network mode> Select LTE/3G/2G auto connect).
2) Search for the Network Manually via following path (Menu> Settings> Connections>More Networks> Mobile Networks> Network Operators> Select Manual Search).
3) Perform the reset network setting (Go to the settings>>general management>>reset>>reset network settings).
4) If still issue persists then we would suggest you to contact your service provider.
For further assistance, kindly visit the nearby Samsung service centre for the physical inspection of the device. The address details of Samsung Service Centres are available at our website. Please click on the link mentioned below to get more information:
