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My gas condensing boiler is set for a temperature of 20° C but to restart it is necessary to wait for the temperature to go down to 18° C. It's unpleasant, I would have preferred that it restarts at 19 or 19.5° C to have a temperature more or less constant in the rooms. I do not know how to do it because the 18° C correspond to the temperature of the night.

You do not specify the age of the installation or that of construction. The recent thermostats, installed in a house with the current standards of insulation, react in a very fine way to the variations of temperature.
The problem may be at the thermostat, or the location of this thermostat, in the case of a radio link.
You can test by moving this thermostat, to position it in the room most sensitive to external temperature variations.
A good solution would be to set up an outdoor sensor, a technique that takes into account the temperature variations outside the house, and refine the control of the boiler.
Install this probe preferably east or west, but never south, as this would distort the information by excessive exposure to sunshine.

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