No cellular service for iphone 6s IOS 15.7.3
Your phone is currently unable to access cellular data or make or receive calls or messages. Was working last night fine. Possibly an auto-update and install?
iPhone 6s
1.
Try to do a force restart -> Force restart iPhone – Apple Support (UK)
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.
2.
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
3.
iPhone 6S is > 7 years old, which also affects the battery life. The hardware remains old, so does the battery.
If it is still possible, have the battery replaced.
The battery can be replaced by Apple or an Apple Authorized Service Provider.
See this Apple article: iPhone Battery Repair & Replacement - Apple Support
If you are going to replace the battery, see -> Get your iPhone, iPad, or iPod touch ready for service
Get your iPhone, iPad, or iPod touch ready for service - Apple Support
The price is approx $ 69.
If you want to know exactly, call Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)
or
call 1-800-MY-APPLE