Just a thought, might it be possible to free the ATC Push Rod by sharp taps in situ? If not remove it.
Not overly convinced that would work or is altogether wise. The ATC actuator is a significantly "powerful" thing, it has to push with quite a force to apply the brakes the 40 odd percent it is designed to. Thus, with that amount of "welly" its most unlikely a seize that would freed by a light tap or two will unstick it.
It possibly might jar a stuck-up end stop switch if they feature such things to park at the withdrawn end?
I would only do any tapping with real care and with the whole being covered by the seller's CRA 2015 obligations, then any intervention is not that wise.
Also, reportedly the push rod can come out, so removing it to test might invite issues with that occurring.
I would check if the fuse in your towcar that supplies the 12 Volt powering has not blown, as already suggested.
Typically, IME these fuses in factory fit are 15Amp, that is not confidently a high enough rating not to risk blowing. 15 Amps can work but can be vulnerable to blowing.
Alko suggest 20Amp [if my memory back 16 years holds}
If that is intact then I would be investigating that powering voltage is getting through each bit right to the ATCs socket, and wiggling them to help ensure the contacts carry the not insignificant Amps involved.