Plusnet 2704N - root password and other questions

There was a terminal-only ROM on this page (the 15.05 version):

which I flashed to the router, it seemed to work okay, now when I try to access the router by connecting with root over SSH directly with a LAN cable (to it asks me for a root password which I don't know. And the page for that router doesn't mention any root password. Some other page on the Openwrt documentation mentions that it should let us set a root password.

Edit: I tried connecting the 2704N to my main ISP router and then connected my laptop to the 2704N and it seems to work okay as a switch which is what I intended to use it for. But at some point it would be good to actually be able to access the shell somehow and be able to configure things (and to update to the newer version of openwrt that is compatible with this device).

Does the default configuration of openwrt just let devices work as a switch?

I guess I have a follow-up question, is it possible to go back to the stock firmware and install the latest available rom (19.07)?
And are there instructions on how to create the ROM since it wasn't clear on the wiki page mentioned above.

Another follow-up question: should I be getting 100mbps with this? It looked to me like it had gigabit ports...