My isp limits all computers and iPads connected to 4 devices. How to bypass?

We are running a large set-up of 10 APs throughout our property. All wifi is through Ubiquiti’s unifi product line, so the router is a Unifi USG Pro 4 port.

I can’t tell how the ISP (China unicom) does this, but its maddening. Are they detecting device signatures to avoid blocking phones, or is it via DNS? I’ve tried switching to alternative DNS addresses to fix this and sometimes this worked. But if I could understand the mechanism for doing this I’d hope to really be able to solve this once and for all. Any ideas??