iPhone losing Cellular

Try first to optimize the range of the mobile network, see this Apple article:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

Check your Wi-Fi Home network, see the following Apple articles:

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

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

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

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

Recommended settings for Wi-Fi routers and access points

Recommended settings for Wi-Fi routers and access points - Apple Support

If you need help with your Wi‑Fi password

If you need help with your Wi‑Fi password - Apple Support

I have this same issue since a recent iOS update - it immediately started happening the day I installed the update. Toggling airplane mode on / off has been the temporary solution for me.

Do you use a virtual or physical sim? What carrier?

When you leave the apartment, Home Wi-Fi is automatically disconnected.

Then you need to turn On Mobile Data or use Public Wi-Fi, see -> this Apple article: Use captive Wi-Fi networks on your iPhone, iPad or iPod touch

Use captive Wi-Fi networks on your iPhone, iPad or iPod touch – Apple Support (UK)

Contact your carrier about your data plan.