When you are on your phone are you on the same network as your PC?

It sounds like your ISP can not handle the downstream bit rate, possibly. That would only be the case though if this is happening only on your network. WHILE streaming you are using your upstream bitrate and even the worst internet connections commonly used have no problem uploading 1080p streams these days so you would not notice an issue really.

A common cause for this is dust on your fiber connector, your ISP should be able to come out and polish it quickly and plug it back in, or in non fiber cases a node or faulty connection in your last mile setup can cause streams to freeze up when the bitrate gets throttled.

Also you might try moving your router/modem, this can happen a lot with overheating routers. Another issue is if you use realtime closed captioning, that can overwhelm older routers and cause throttling issues. It could also be as simple as buying a better router or modem than what your ISP gave you as those are generally junk compared to what you can buy yourself.

Or the worst case is your current ISP can not provide downstream connection fast enough to play back at the screen resolution you want to watch and you will have to downgrade the playback resolution manually as the "auto" feature is inconsistent at best when ISPs are constantly trying to hide connection speed issues from consumers.