I want to use my cellular data to update my iOS?

Sorry no.

To perform an update procedure wirelessly, you need to connect your device to a reliable and fast Wifi Network with internet access (no cellular data, unless you use a 5G network, no personal hotspot on your device, no VPN connections, no low power mode are admitted). AND I suggest that you keep the device plugged into power during the whole update process.

Do not forget to create a full backup of your device, before any updates.

Available wifi networks cannot be seen in Settings>Wifi? Does it happen at home only, or with all different wifi networks too?

Try to reset your iPhone Network Settings, by going to Settings>General>Transfer or Reset [Device]>Reset> Reset

Network Settings, bearing in mind that this procedure also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before, but no other data is affected. When done, restart your iPhone.

If the issue persists, I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region . After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.