Issue with Twitter and my router

I’ve been having a weird issue with Twitter for the past few weeks, and surprisingly, I managed to pin it down to my router.

For weeks, most images on Twitter wouldn’t load, both on my PC and my mobile. I assumed it was something on their end, but it lasted for too long. Tonight I tried to ping pbs.twimg.com, which is where an image that wasn’t being loaded was hosted, and it was not reachable.

Both my desktop and my mobile use my home connection, and when I switched to my data plan on my phone, magically all images on Twitter would load. My girlfriend never had this issue on her Mac or her phone, because she was using her own data plan; when she tried to switch over to our home connection, she got the same issue. She doesn’t get this issue if she connects to her work VPN while using our home connection. If she uses the VPN or her own data plan, pbs.twimg.com can be pinged with no issues.

Our home connection passes through a router. The router gets the connection via an Ethernet cable. My desktop is connected straight to the router through another Ethernet cable. If I remove the router from the equation and plug the Ethernet cable from the wall straight into my desktop’s NIC, all images on Twitter load just fine. Hence, it must be the router.

This issue only manifests with images on Twitter. It’s not a big deal, but if possible I’d like to fix it, except I wouldn’t even know what settings in the router config I should change. I did check if there wasn’t any weird content filtering or firewall setting on, but it doesn’t look like that. Any other ideas? The router is a Zyxel NBG-418N v2 (It’s a fairly old model but I doubt it boils down to this.)