@aaPanel_Kern server was accessible and running well. All websites running well too. All ports accessible and working well too. There are no restrictions in place.
If I was accessing it well before for months, it means aaPanel port was working okay.
I remember there's a time someone posted about the same problem on this forum and couldn't get any help.
I was forced to rebuild the server. Even restarting the server wasn't solving the issue.
I didn't know about restarting aaPanel. I will try that if it shows up a again in future.