Why there is no real power difference when road is climbing, from 0 up to 5 is quiet the same , them 6 to 9, and over ?

I have an old (2017 I think) Tax Flux. Never really used it before but now I tried to do some workouts and I think I have the same issue. I updated the firmware first since it was relly old. Now there seems to be no change when using power/ERG mode and in slope/climb mode there seems to be a pretty big difference between 5% and 6% but nothing before that. It does give power, speed and cadence readings via Ant+ and it seems to connect fine with the Tacx Utilities and the Tacx Training app on Android over Bluetooth, the connection is less stable with one of my two phones but the other seems really fine.

I've tried the Xert App as well (that's what I was hoping to use) over Bluetooth and over Ant+ (one of the Phones has build in Ant+) but I don't feel a reaction to anything. No error messages either, everything seems fine but there is no control of the Flux in ERG mode. It lets me do any power I like just limited to what my old and completely untrained legs are capable of.

Any ideas? Firmware is up to date, the Utilities App doesn't see any problem, the trainer seems fine, lights behaving as they should I think. Bluetooth and Ant showing connection if any and the power I think blinking green when on and awake. Nothing that suggests an error state.

I've tried it again. At first it at least was usable in slope mode at 0%. If felt more or less like a bike on no slope should feel. It said about 80 Watt or so when easy padaling that would maybe be 15-20 kph outside. That is reasonable, right?

After 5 min I decided to calibrate it again and see if that fixed anything. It made it worse. Now as soon as I start the slope mode on 0% it becomes unreasonable hard to even pedal at all. Says at least 150 Watt, easy over 200 W as soon as the pedals turn, that's nothing that I can keep up for any amount of time and I can ride a bike in a medium gear on no slope for half an hour at 20 kph without keeling over, I have done it recently. That's pretty much what everybody who is no athlete and not sick or seriously out of shape either can do, I'd say I'm in that category.

It still works when no control app is running, gives the about 80 Watt reading and feels more or less normal, even if that's hard to tell for me.

I'm confused

Any help would be appreciated.