Restore license on iPhone

Open Pianoteq app on the iPhone.
At top right, click Options.
Go to Instruments section.
Click Restore purchases.

That should do it, I think.

In the Options, About section, there is also a button "Update License".
For me this always redirects to the instrument section (as above). But perhaps if you haven't yet activated pianoteq on iOS you may need to click this one first.