I just traveled from Egypt to Qatar and my iPhone just can't catch any signal at ll

If you are traveling you need Mobile Data or Wi-Fi network, see this Apple article: Use captive Wi-Fi networks on your iPhone, iPad or iPod touch

https://support.apple.com/en-gb/HT204497

You always need Wi-Fi or Mobile Data, if you're not on Wi-Fi, it will use mobile data.

Check your Mobiel Data plan from your carrier, or use a local prepaid card when there is no Wi-Fi network.

Follow 👉🏼 this helpful guide here. 

You have to 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