KDE

You could restructure your network, but it's probably not necessary. My phone is always behind NAT on the secondary router's wi-fi. I got it working by:

• Reserving a static IP address for my phone in the secondary router's DHCP server.
• Forwarding that router's incoming ports 1714-1764 to the phone's IP address.
• Allowing outgoing traffic from that router's network to ports 1714-1764 on my workstation's IP address (on the primary network).
• Adding a device (my workstation) by IP address in my phone's KDE Connect app.

For one thing, you need to bridge your 2nd router and disable its DHCP server, effectively making it an Access Point.

If that's not enough to address your issue, you could also try setting the Wifi SSID and password the same, provided its set to use different channels than your main router, although this can make it a pain to force a device to switch from one router to another. (my second router also has a separate SSID partially because the one for the kids' devices and living-room TV shuts off for bed-time).

End of the day, unless you have commercial routers/APs or Mesh routers that are setup to inerroperate on the same SSID while using an Ethernet backhaul, there's going to be quirks and compromises.

Why can't you just use a switch and an access point?

Why do you have 2 routers.