My iPhone 7plus is still showing cellular issues detected after an update, pls what do I do

Did you already try the troubleshooting mentioned in this related article?

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You need a computer to follow these steps.

1. Make sure that you have a backup of your iPhone or iPad.
2. Connect your device to a computer that has the latest version of macOS or iTunes. On a Mac with macOS Catalina or later, open a Finder window. On a Mac with macOS Mojave or earlier, or on a Windows PC, open iTunes.
3. If "Trust this Computer" appears, unlock your device, then tap Trust.
4. Select your device.
5. In the Finder, click General. Or in iTunes, click Summary. Then:

• • On a Mac: Command-click Check for Update.
  • On a Windows computer: Control-click Check for Update.

6 . Click Download and Update. Continue by following the onscreen instructions.

Your computer will download and reinstall the current version of software on your device. Don't disconnect your device from your computer until the download completes.

If the message still appears, contact Apple Support.

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copied from: If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart - Apple Support

iPhone 7 Plus, iOS 15

Troubleshooting - iPhone 7 

1. 

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

If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart - Apple Support

3. 

iPhone 7 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

note:

You have to decide for yourself whether it is still worth having the battery replaced.

This exact same thing is happening to me. My Verizon phone service has always been rock solid. I have done ALL the recommended actions (I have been on dozens of Apple Iphone community reports like this one) , the last one is to try a new SIM card (which I just ordered from Verizon) but I am skeptical it will fix this.

VERY disappointed, it seems like Apple and Verizon are creating a forced obsolescence of the iphone 7 ! If thats true, I'll just go Android !