Simply buy Cubase Artist from Steinberg and you’ll still get the deal (until 28th), and use your Thomann credit for something else.
Since you did not buy from Steinberg you have to activate the license using the USB e-licenser at latest the 28th, as clearly stated.
If you had bought if from Steinberg instead you would get the deal, now you missed it (unless you can get your hands on an USB e-licenser in time).