Every published troubleshooting guide, with the newest first.
Mobile network not available means the phone cannot reach your carrier. Check coverage and the selected SIM, reselect the network, reset the APN, then reseat the SIM and…
A phone that keeps losing service has a coverage, network or phone problem. Work out which first, then toggle airplane mode, reseat the SIM, change the network mode…
An Android phone that will not charge is usually the cable, adapter or a dirty port. Test these first, then check battery settings, safe mode and the hardware.
Android shows Wi-Fi connected but nothing loads? First decide whether the network or the phone is at fault, then work through these fixes.