Cellular Data on Iphone 8 Plus

Are you using a case that can interfere with your cellular signal? if yes, try to remove it.

Did you already tried a forced restart of your device?

On iP8+: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Side button until you see the Apple logo. When it appears, release the Side button, wait for your device to restart, then test it.

Regards

Giulio

Talk to your carrier and see if they can help you.

Regards

Giulio

I don't think that iOS 12.3 is involved.

As already suggested I would try and contact your carrier explaining your problem.

Regards

Giulio