Why does my iMessage keep saying waiting for activation?

To activate iMessage and FaceTime using your Phone number

First, check to know whether you have an SMS plan enabled at least for One SMS with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you

If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS Only)

If you can't turn on or sign in to iMessage or FaceTime - Apple ...

Check also:

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

Low Power Mode is On. Charge the iPhone.

Wait until the battery is fully charged (100%).

The Low Power Mode will turned off automatically.