Facebook/Instagram not working on wifi

A

AC Question

Hi guys, i'm running into this strange issue:

I have Netgear R7000 router, connected through my ISP cable modem, using basic default settings for wifi.

For the past 2 days, wifi is working fine however​ i'm unable to use Facebook/Instagram on 3 android phones (S6/S8), app opens but will not download/display or update the feed/pictures/videos on the Facebook/Instagram app while on wifi.

Once i turn off wifi, all works fine, works fine on 4g - cell data, but no good on phones on wifi. I tried uninstalling apps/clearing cache no luck.

I thought of installing a free vpn from playstore, and run the apps on wifi while running vpn, everything works fine. Feed, videos, pictures are updating, and working as normal.

I haven't changed any settings for router. So no idea what caused these apps to be unresponsive on wifi for the past 2 days.

I did some research and it seems to maybe boil down to IPv6 settings for router. I read up to try turning the IPv6 off in the router, mine was already off.

Anyone have any idea what could be the issue here? Thanks