Cannot see icloud and Apple share album photos with Xfinity internet

Since 3 days ago, I couldn't see any photo or video from icloud or Apple shared album.  All other websites and streaming services are working fine. I can log in to icloud but when I go into "photo" menu all thumbnails are blank and photo/videos cannot be loaded.

I tried a lot and found that,

1) Modem - Router - Devices (Ipad / PC, Iphone) --> Not Working

2) Modem - PC (connected with Lan Cable)  ---> Working

3) Modem - Laptop(as a wifi hotspot)  - Devices (Ipad / PC, Iphone) --> Working

 

I tried with multiple different routers but the problem is all the same.

A) As it's working fine with option 2), 3), and with Cellular data from my phone, Apple can be ruled out from possible root cause.

B) Since the problem is all the same for different routers from different manufacturers, I don't think it's a problem caused by router.

 

So my conclusion is Xfinity put a certain restriction. Maybe to drive me to use Xfinity's router? (Although I haven't tried with Xfinity's router or Modem/Router all-in-one thing.)

I spent 2 hours with 5 different Xfinity CS staffs but sadly they were all clueless.

Does anybody have a clue?