You have to press and hold the playsi button on the steering wheel before you put the cable in the playsi then you put the cable in the playsi and it should work.
The steering wheel is not directly compatible with Forza Horizon 5. However, there are ways to use it via emulators, as described in some instructions. However, other steering wheels from manufacturers such as Logitech and Fanatec are officially supported for Forza Horizon 5 on PC and Xbox.
The "T80 Ferrari 488 GTB" steering wheel from Thrustmaster is not directly compatible with Forza Horizon 5, but there are ways to use it with emulators such as vJoy and Forza EmuWheel to integrate it into Forza Horizon 5.
Without wanting to be a great expert, I would say that the thing seems to be defective. If the rest works, it won't be the power supply unit. I assume you have already tried plugging it in and unplugging it?
My grandson for whom the steering wheel was bought wrote :
The steering wheel is not supported by Gran Turismo 7 at the moment, maybe in the distant future. I recommend the Thrustmaster T248 steering wheel if you want to have a steering wheel that will certainly be up-to-date for a few years to come.
I have never had this problem. What I sometimes had was that the straight run-out was not correct and I unplugged the PC plug and plugged it in again and it was OK again.
According to my research, the T300 RS is very similar in features:
Thrustmaster T300 RS
Unfortunately, I don't understand exactly what you mean by compatibility with another Thrustmaster steering wheel - However, Thrustmaster pedals have the same connector and are usually compatible with each other 👍
The manufacturer does not seem to give any information on this.
However, I assume that this should be possible. If not, you should be able to return the item if necessary.