iPad Pro (12.9 inch) connected to R7000P not working with certain app

I recently moved and xfinity made me get a new modem. I never had a problem for the last year at my last house. Then I moved and had some issues but buffed them out. However, two days ago my iPad started giving me issues.

There is an app and website (MTNTOUGH Fitness) that no longer plays videos when connected to my wifi network through the R7000P. Furthermore the iPad doesn't seem to connect to my Apple ID when connected to my wifi network. If I connect my iPad to a wireless hotspot via my phone everything works fine. Therefore I'm pretty sure it's a router/iPad issue. 

To make things more confusing, my iPhone 12 Plus plays videos from the MTNTOUGH app and website just fine when connected to the wifi network. 

There are other issues with the iPad when connected to the wifi network as well:

Mail app doesn't update

Can't turn on/off iCloud services (e.g. find my iPad)

Can't download any apps at all

Remedies I've tried:

Restarting modem

Restarting router

Factory resetting router