My phone is saying an update is required to use cellular data. But when I go to update my phone it says I can’t update because I’m not on Wi-Fi. When in fact I am connected to the Wi-Fi. What should I do?

Cellular Update Failed —> If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart

If you are unable to make a call or receive; If you can't make or receive calls on your iPhone - Apple Support

Check your WiFi Router / Modem for --> Recommended settings for Wi-Fi routers and access points ...

Connecting to Wifi is the first part and then getting connected to the Internet is the second

1. If you connect to Wifi, doesn't necessarily mean you are connected to the internet. If you are not connected to wifi could be a problem with iPhone settings or Router/Modem Settings
2. If you are not connected to the internet is the sole issue of Router / Modem Settings. Contact your ISP immediately

For your Internet connection.

• Over Cellular: contact your Carrier (Cellular Service Provider). The agency who issued the SIM to you
• Over WiFI: Contact your ISP (Internet Service Provider), the agency who provided the Internet Service on your Modem/Router to you

However --> Connect iPhone to the Internet - Apple Support