[Solved] Brotje Gas Boiler Error on Volt UPS Power - Phase and Neutral Issue

I have the following situation.
- I have a gas cooker, which of course needs electricity to operate (like everything today).
- I have a UPS with a large battery (UPS from VOLT) that generates a pure sine wave,
- The UPS is permanently plugged into the distribution board between the grid and the cooker (accidental pole swapping is not possible),
- The power input to the UPS is protected by overcurrent protection and RCD.
- The cooker and UPS are correctly earthed.

When switching to emergency power, the cooker reports an error of no ground connection and stops operating. From information obtained from the cooker manufacturer, the error message is not quite correct. The cooker needs a designated and invariable phase and neutral (and of course earth) to operate. This is because it uses the potential difference between phase and earth for flame detection (conduction under ionised flame conditions) and for this the potential difference must be > 150V.
When switching to UPS power, the concepts of phase and neutral disappear, because then there is only a potential difference between them and no reference to earth. Thus the furnace ceases to function .

I have therefore decided to connect the output from the UPS to the primary winding of the isolation transformer, and connect the secondary winding to the furnace but with the proviso that one of the poles of the secondary winding be connected to earth and this will be the neutral pole in the furnace. In this way, the reference to earth will be maintained.
In addition, I am planning an overcurrent protection device connected to the RCD between the cooker and the secondary winding in order to maintain further protection.

Now the question is, have I forgotten something and does all this make sense?

PS
I deliberately don't use the resistor method between N and PE because somehow I'm not convinced by it and I have the impression that then I would be violating the correct configuration of my whole local network.

The idea is quite OK, but it would be simpler and cheaper to install a resistor between N and earth (PE) behind the UPS:
https://www.elektroda.pl/rtvforum/topic358710.html

As I wrote, I'm not convinced about the resistor solution because I have the impression that I'm changing the grid layout this way. So I used that solution presented in the first post with grounding one of the poles of the isolation transformer and defining it as the new neutral for the cooker. Since then the whole thing has worked smoothly and the furnace has stopped reporting error messages when operating from the UPS.