@KawaBoy 

Hi,
It seems the Tapo camera is connected to the Wi-Fi network but has no internet access. Please check if your Wi-Fi network is working.
If yes, you can check if there are Firewall or Access Control entries configured for the camera on your router so the camera has no internet access.

If the problem remains, please help confirm the following information.
1. What's the LED status on the camera? Such as solid red, flashing red, etc.

2. Please describe your network diagram, such as the Internet--wireless router))) Tapo Cam.
What's the model number of your router? Is there an AP/RE between the router and the camera?

Best Regards

  @KawaBoy 

Hi,
Thank you for the case sharing.

Firstly, if you can access the camera when the phone is connected to your friend's Wi-Fi, it means the camera is working fine and can be accessed remotely. 
Your GSM network can't access the camera maybe the SIM card service provider has limitations or assigns improper parameters, such as DNS server, and rebooting the phone has obtained new parameters thus the phone can watch the camera again.

Best Regards