Hi, friend!
I noticed that you described the ONT resetting behavior.
You need to log in to the ONT web page to check whether the WAN configuration is lost.
The possible cause is that the WAN configuration is lost after the ONT is reset to the default value.
If the ONT does not have a route WAN port, the ONT does not automatically dial up. As a result, the dialup fails.
In this case, you need to configure the WAN for the ONT to make the dialup successful.
Thanks!