I guess your Arris CM820 is a modem, not a router. Switch has to connect to a router, not a modem. So you need to buy a router.

What's the model of your switch? You can go to tp-llink.com to search for your model and go to support page to get related documents. Here is a QoS configuration guidance for T2600G-52TS:...

You can log in to the web UI and change a new login username/password, or change it back to admin/admin. Or just reset it directly. I guess static DHCP leases can't be migrated. Address reservation...

@Semaj26 Usually, CPE is used to build point-to-point transmission. You can read TP-Link's official guide:...

If EAP225 connecting to the switch works fine, how did you know the switch disconnected? What are the specific symptoms of disconnecting?

I think you can set the IP addresses of both switches to a different subnet. i.e. router's LAN IP is 192.168.0.1, you can change the switch's IP to 192.168.2.1 or something else. Thus, the wireless...