Help with serial access on new device

Hello, I'm trying to make progress on this device: Sercomm SHG3000
https://openwrt.org/toh/vodafone/vodafone_power_station

I got a serial console, but it only accepts input at some points. For instance, when it launches it doesn't accept inputs, so I can't interrupt boot.

*** Press any key to stop auto run (1 seconds) ***
Auto run second count down: 110

But it starts echoing back keystrokes after this:

Registering SWP/SWPB emulation handler

It doesn't accept commands though, it just echoes back keystrokes for a while.

Then it gets to the final output, and after 30-40 seconds it stops echoing keystrokes again (serial goes dead).

This is strange behavior. The fact that at least sometimes I do get keystrokes echoed back leads me to think that there is no hardware issue. Could it be that the serial gets enabled and disabled in software?
Has anyone encountered this before? Any advice?

I've got serial on a few devices in the past and they either always accept serial input or never, this is a first for me!

something you can try is get the oldest firmware you can find
back date the device and see if it's changed at all