Help me I updated my phone to iOS15 So my phone just lost a network

1.

Optimize the range of the mobile network.

See this Apple article: If you see No Service or Searching on your iPhone or iPad

https://support.apple.com/en-us/HT201415

2.

You need strong Wi-Fi, see the following Apple articles:

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

https://support.apple.com/en-us/HT204051

Connect to Wi-Fi on your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT202639

3.

See also the following Apple articles:

Update your iPhone, iPad, or iPod touch

https://support.apple.com/en-in/HT204204

If your iPhone, iPad or iPod touch won’t update

https://support.apple.com/en-us/HT201435

4.

You need about 6 GB device storage for update.