@Sunshine 

Hello, thank you for your reply. 

I followed your instructions and was able to isolate the pairs of Powerline adapters from each other, however upon logging onto my PC today, I can see that one ISPs connection has overridden the other, and now I am connected to the wrong ISP via the powerline device in my office. 

As far as I am aware, no Powerline devices have been unplugged or re-paired with any other devices. 

Is there a way via software to make sure that this doesn't happen? 

  @georget2023 

Hi,

What does the tpPLC Utility say? Are the two separate pairs of Powerline adapters still connected the same way as the day before?

In case they have somehow cross-connected themselves, then maybe do Sunshine's pairing procedure once more, but this time before you start first reset all four powerline adapters to factory defaults! (press and hold the Pair button for more than 6 seconds)

And maybe also note down the serial numbers or MAC addresses of the Powerline adapters and which one is connected to which modem and located in which room, so that you would always be able to confirm that every Powerline adapter is still plugged where it belongs to.

  @georget2023 You know you can run multiple IP networks across one and the same physical network?

In other words, set your two routers up to use separate IP address ranges, make your endpoint devices use their designated IP range and not the other. That way you won't need two separate physical networks in the house, all the PLC adapters can join one PLC group.

(Yes technically, multiple PLC networks can coexist on one power domain. That doesn't work too well unless the PLC configuration tools expose the option of manual frequency band allocation - and I've only seen that once, in Devolo's long gone "Pro" series. So don't bother.)