Video resolution on YouTube app drops significantly after a couple of seconds. Help!

This issue began yesterday on all three of my Roku devices. When a video begins playback, it is at least 1080p. However, after 2-4 seconds, the resolution drops to what looks like 60p. This happens on every channel. I have YouTube Premium and watch several specific channels often and they are recorded in 4K. Although my TV is max 1080p, the videos have always played at that resolution; images are sharp. Yesterday, though, the resolution drop began. 

Nothing else has changed. I speed tested my ISP and it averages 100mbs down and 80mps up. There is only a single device streaming at a given time, as well. The three Roku devices I have are two Roku Express' and one Roku Premier and this problem happens on each unit.

I spoke with my ISP this morning (Frontier) and they said they found no issues with my signal or equipment. I deleted the YouTube app from my Rokus, performed a hard reset on each Roku device, reinstalled YouTube, and ensured the app was up to date. 

Each Roku's software is up to date and each shows excellent signal strength when running Check Connection, and the download speed averaged 38mps (my devices only run on 2G).

As I stated earlier, the video resolution has been perfect up to yesterday, but I did not change anything and my ISP didn't change anything. And Roku does not provide support for the units I have (nice customer service.)

I would greatly appreciate someone helping here because I do not know where else to turn and the image quality I'm getting now is horrible.

Thanks!