ARRIS TG1652S-85 - how to take it OUT of bridge mode?

Hi,
I have this Arris cable modem/router, which is currently works in "almost dumb" mode - DNS, DHCP, WIFI all are handled by other components of my network, so it does modem and (rather badly) router/NAT functions only.

I want to have a PI4 based OpenWRT device as a router.

I know how to bring Arris to bridge mode, and how to get PI4 to act as a router (connect USB-based adapter as "wan" interface with dhcp to port 1 of Arris, and built-in NIC to the switch).

The question is how to revert it back.

Currently I have set custom password, and Arris IP is 192.168.27.254.
When Arris is a "modem" only - how can I contact it remotely ?
On 192.168.0.1?, or 192.168.100.1 ? From any ethernet port, or it must be port 1 ? Will it as for its current password, or the default ("password") ?

I understand that in worst case I can just reset it, then go to 192.168.0.1 and login with admin/"password", but what can be done without resetting it ?