Manually Selection of Network Resets when restarting phone

[Android Central Question]

I have a Xiaomi Mi A2 Lite which runs Android One (version 9).

I want to manually select a network instead of using automatic selection.
This works fine, but when I restart the phone automatic selection is selected again.

How do I force the phone to continue using the manually selected network even once it has been restarted?

Thanks so much in advance!