Your OnePlus 6T is now fully set up with LineageOS 22.2 + Google Apps, and the Tello SIM is connected to LTE. If the issue recurs after a reboot, just dial *#*#4636#*#* on the phone to open the Radio Info screen and turn the radio on again.
1. Look for "Preferred Network Type" dropdown — set it to LTE/WCDMA/GSM auto
2. Look for a "Turn on Radio" or "Radio Power" button — tap it to turn the radio ON