Internet Issues - Phones and Tablets Connected to Wifi but not Loading certain Websites or Apps

We recently got a new router, and we've been having connectivity issues. My work computer connected no problem, and we got our Alexa and pet camera to connect, but we've had issues with getting other devices to connect and work properly. We had to hardwire our TV and the Playstation to get those to work. Our main issue is that our phones and tablets say they're connected to the internet, but certain websites and apps will not open or load. They will only load when I turn off the wifi and use my celular data or when I hook our tablets up to my personal hotspot. I have not had this issue when I've connected to other Wifi outside of my home network. I've even noticed that my Xfinity app won't load when connected to the wifi and will prompt me to connect to cellular data to access the app. 

I have tried the following troubleshooting which has been unsuccessful: 

- Power cycling the router

- Restarting the router 

- Turning off Advanced Security 

- Doing a network reset to our phones and tablets 

- Making sure private relay is off on my iPhone

- Deleting and reinstalling apps 

- Restarting our phones and tablets 

- Clearing the cache 

- Splitting the wifi bands (that actually resulted in complete disconnection from wifi)

- Turning off 5GHz band and only using the 2 GHz band and vice versa 

- Forgetting our network and re-adding it 

- Ensured VPNs were disconnected 

- Made sure mobile data was turned on with our phones 

- Turned our devices on airplane mode and then turned airplane mode off 

I am not sure what is causing this issue. I am assuming something is blocking certain websites and apps. Has anyone else had this issue? If so, what have you done to fix it?