Netgear N450 and N300 not working together properly

I'm using a Netgear N450 Modem/Router along with a Netgear N300 extender, and have recently had some strange connectivity issues. 

Certain data appears to be blocked when connected to the origin router (the N450). For example, Instagram will load but not Twitter or Snapchat; text messages in iMessage can be sent/received but photos cannot; only certain websites won't load... it's almost like there is some sort of specific filter in action. 

However, when I switch to the extended network (N300), all the data comes through without any problem. 

It's strange to me that the origin source would be the problem network as opposed to the extended network... since there's an extra layer of complexity there. I assume that this issue has something to do with me frequently switching between the two networks - perhaps I'm trying to fetch data from the wrong IP or something? 

I've tried isolating the issue by unplugging the N300 extender, but I still have the data issues with the main N450 router. I've also tried the basic "unplug and wait" routine, of course. 

Any idea what could be going wrong?