Thanks guys for the support so far, truly a helpful and strong community.
de0u already configured the phone settings to per-network randomized MAC
on both wifi frequencies but didn't reboot the router yet. Will give it a try and see if the problem persists.
r134a Apologies, in my last response i've asked if u tried https:// i just see u did. I've started typing when that comment wasn't there yet, and received a phone call, then finished typing and submit :)
no worries (:
I do have another server running in the same machine and it does happen to share the same problem of the phone not being able to reach it sometimes.
At this point I'm suspecting this is not related to grapheneOS but rather something on my network as you mentioned. Got the same problem on wife's iPhone, but not on other laptops and tablets we have.