KDE Connect - connection across routers

I have two routers in my network, one master router and another supplementary one that extends the main one using a LAN cable.

My desktop and laptop are always connected to the main router, but my phone sometimes connects to the other, due to me moving about the house. I keep having to manually connect back to the main router when I need to use KDE connect. I don't want to 'Forget' the second router as it is the only one that covers certain parts of my house, such as the kitchen.

Is there a way around this? I had earlier connected the second router to the main one without a cable (wireless extender) and KDE Connect worked well in that config. I'm sure I'll need to provide some more info about my router setups, but I'm not sure what is needed, I can give more context in response to queries.