iPhone SE 1st gen - September 12, 2018 -> you can update to iOS 15.7.5, see:
iPhone models compatible with iOS 15.5 – Apple Support (UK)
iPhone SE 1st gen can’t be update to a higher iOS -iOS 16.4.1.
iPhone SE 2nd gen - April 24, 2020 & iPhone SE 3 rd gen - March 8, 2022 -> you can update to iOS 16.4.1, see:
iPhone models compatible with iOS 16 – Apple Support (UK)
Only Apple knows how long iPhone SE 2nd gen and 3rd gen will be supported by Apple.
If you have 'no service', see these Apple articles below:
If you see No Service, Searching, or SOS on your iPhone or iPad
If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support
Manually update your carrier settings on your iPhone or iPad
Manually update your carrier settings on your iPhone or iPad - Apple Support
Update your network provider settings manually on your iPhone or iPad
Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)
If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart
If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart - Apple Support
If it doesn't help, remove and reinsert the SIM card.
If it doesn't help visit your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.
Ask also to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.
If VPN is On, turn Off VPN.