Even when you get sound out of your combo, I’d be surprised if you find long-term satisfaction from this setup.
112 is the most entry level pre that Naim have ever released; the Flatcap is the minimum quality psu for getting any sound from it whatsoever.
The 250 is pretty near the top of the pile for power amps.
This is the wrong way round! A top level pre fed with a satisfactory source, even with a low level power amp, will get the best performance from that power amp and probably sound really good. But a 250 fed by a poor source / pre will probably sound harsh and muddled.
If you can afford to, sell the 112 and replace with 32.5, 72 or better - you’ll thank me later.
It may not be an ideal combination, but if all is well then the OP should at least get some sound out of it all, so first let’s try to get to the bottom of why it isn’t working.
Edit, I’m all good now, I tried all three DIN4 ports on the Flatcap, The one closest to the DIN5 works, the other two appear to not work ( for this function at least, maybe they have a different function.