Eventually I could upgrade the wi-fi module. It seems the upgrade only works with the master fixture and previously I had been trying it with one of the slaves.

Once I upgraded the wi-fi module I could set up the wi-fi and get an IP address. Afterwards I could connect to the fixture by means of the app and everything showed up there.

Well, so far so good. Apparently I do not need to upgrade the firmware but should I have to do it, I guess I can do it by means of the wi-fi connection.