Galaxy S6 :: Not Receiving Some App Alerts On Home WiFi?

I've had a Galaxy S6 for about a month now, and for the past two weeks I've been fighting tooth and nail with it to try to fix the issue in the title. I believe (based off what little I've learned from Googling this up) that it's an issue with apps that use push notifications (GCM?) alerts, and includes Facebook messenger, Google Hangouts, Steam chat, TheScore (sports app), and some others. If I'm on my home wi-fi, those alerts MIGHT show up, but 95% likely won't; as soon as I disconnect from my home wifi, I'll get a deluge of alerts that had been 'backed up' and waiting for the chance to be delivered.

This does not happen while on cellular data, and it does not happen on other wi-fi networks from the same ISP.

After Googling the heck out of this, things I have tried include:

* Making sure that the 'keep wi-fi on while asleep' setting is set to always.
* Adding the affected apps to the priority list.
* Having a Samsung employee at Best Buy reinstall the OS. (I think that's what she said she did.)
* Changing my router to a static DNS, pointed at 8.8.8.8 and 8.8.4.4.
* Downloading that DNSet app, though I quickly deleted it both because it didn't seem like it was working, and because I didn't savor the idea of an app sitting in the middle of my data like that.

The handful of other threads I've found with a similar issue have all seemed to devolve down to a single thought: Lollipop (perhaps even specifically Lollipop on Verizon Wireless devices) does not always play well with IPv6 data, though it's entirely possible I've settled on that as the culprit just because it makes me feel better having a specific target in mind.

My router (Netgear C6300) does not seem to have a setting to specifically turn off IPv6 data. Any way to specifically turn off IPv6 usage on a Galaxy S6 without rooting the device?