Was about to ask the same question. Speedtest speed is okay. Just YouTube is buffering. No disconnect though.

USB tethering from Android phone. No slowness with VPN. 

No issue on Google DNS though. Have you tried Google DNS?

Btw, no issue to play 4K video at all

Yes, this is what i want to mean.

When network is congested, prioritize other users first. If no congested, share equal bandwidth.
If found high usage customers, check his background. If he spend on illegal activities, ban the server, or ban the user account.

Not simply judge or ban user simply because he spend X amount of data without proving what did he do.
This is not unlimited plan.

Look like is not device compatibility issue on my case. Changed new router. Good for almost a day without any issue.

Yes, YouTube buffering and ping timed out issues are back again. 

Yes, same spot.

I'm thinking of buying either Huawei B310 or E5186. B310 is cheaper but only CAT4. While E5186 is more expensive, but supports faster CAT6. I have a question for those using B310, are u satisfied with it? Or would you prefer to use E5186? I would be happy if I can get at least 10Mbps.

Right now I'm using Huawei E5172-515s which only supports band 2, 5 and 7. Speed is a bit unsatisfying at around 3-5 Mbps. Correct me if I'm wrong, but as Digi has LTE on band 3 and 7, my current router cannot benefit from carrier aggregation.