@Silenos 

If your Main Deco is currently connected to the LAN port of Plusnet Hub One modem/router, it should be as simple as connecting Main Deco to the LAN port of Fritz!box 7530.

Make sure you have wifi disabled on Fritz!box 7530, just like you have it with Plusnet.

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You didn't tell if you run your Deco mesh in its default Router or in Access Point mode, but when you have ISP router you should consider running Deco mesh in AP mode: Switching a Deco to Access Point (AP) Mode

In that mode Main Deco becomes just another connected wired device as far as ISP Router concerned, just like desktop PC. And just like desktop PC, you plug Main Deco that is in Access Point mode to a router, any router, and it should work.