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

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?

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:

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?