You may have noticed a setting on Android called Private DNS. It's on by default, and you're best off leaving it that way.

Mobile devices sometimes get lost. A laptop bag gets left on the bus or train, a smartphone slips out of your pocket, or a USB flash drive falls to the ground unnoticed. Losing a notebook or phone ...

You can't change your phone number in the Signal app directly, but you can deregister your old number and then update the app ...