Nest Thermostat - W wire not detected

Hi all, I have just recently wired up my Nest Thermostat to my old Braemar TH3.

As I don't have a C wire and have done a thesis amount of research, I bought a 24v transformer instead and wired it into C and R. The two wires from the Braemar have been wired into W and R (combined with the R wire from the transformer).
Issue now is, that the Nest Thermostat throws an error n253 – 'W wire not detected' and continuously turns on the heater. When I unplug the 24v transformer, the Nest detects the W wire but not the R and C wires, so W wire is present. Does anyone know how to fix my issue? Are my thermostat wires too thin?

Thanks :)

Submitted by Icy_Statistician_641

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