Zyxel C3000Z - Found to be defective, Need help securing my network?

Networking & Firewalls
1

I bought this modem years back, yesterday CentryLink had me do a forced firmware update to be able to connect, the update does not allow me to disable the 5G network, hide the SSID nor change the password. I don’t want to use it at all, in fact I’m searching to replace this modem with a wired only modem, I’m using tp-link’s EAP225-Wall.

So I want to open the case and disconnect the wireless circuit but, I can not find the board layout. 1) does anyone now where I can find one or 2) direct me how to disconnect it without disabling the modem?

2

Just spoke with CentryLink and after further diagnostic with the Tech, its found that my modem is defective. So I have to order a new one anyway but, until then I need help in securing my network from the possibility of discovery of this weak default password, being I’m unable to change it. My network looks like this:
TL-R600VPN

I’m thinking I can put my TL-R600VPN between the C3000Z and pfSense and deny any LAN connections from the C3000Z, but I need help on how to do this or could this be done by pfSense without the TL-R600VPN?