Belkin router F6D4230-4 v1 not letting Facebook app connect

Last week I started having problems with Facebook on both my Nexus 7 tablet and my Nexus phone when connected to my Wi-Fi router.
I haven't been able to find a solution on Google, and restarting my devices, router and modern hasn't helped.
Symptoms (both are intermittent, but 'more often than not':

• Facebook groups often won't load. Click the group link and it goes back to news feed. 
• Facebook notification list won't load. Clocks for a while and then says 'Couldn't fetch notifications'

however

• Facebook news feed loads and refreshes fine. 
• Facebook chat works fine. 

The kicker is that the symptoms are very similar in both the app and the mobile chrome web page from the tablet and phone. The second kicker is that on my computer, also connected via Wi-Fi and running chrome, Facebook works perfectly.

My phone not on Wi-Fi works fine. My tablet connected to my phone hot spot works fine.

Since the devices work fine not on Wi-Fi, I'm assuming they are working correctly. Since the computer works fine on Wi-Fi, and the devices worked fine on it last week, I'm assuming the Wi-Fi set up is correct. I'm at a loss.

My set up:

• modern is a Top Global MB8000 wired to the router 
• router is an aging Belkin F6D4230-4 v1000  with disabled security. Toggling the firewall off doesn't help. Putting the tablet int he router's DMZ doesn't help

My guess is that the app uses different network connections than the web site, and those connections are intermittently getting whacked (technical term) by the router. Current workaround is to use the MB8000's Wi-Fi instead of the router for the mobile devices.

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