Grundig tech probably does not know what products are made by others to solve your problem. But they probably do know the modulating frequency of the IR signal. Evidently the remotes you have tried can learn this frequency but it is outside of the frequency range of the IR repeater.

Please contact Grundig again and ask about this. For reference, most audio/video equipment uses a modulating frequency in the range of 38 to 40 KHz. If the frequency is not in that range, someone can probably make a suggestion.

Also, send an email to the tech people at www.xantech.com. If they are familiar with Grundig's signals, they might be able to suggest one of their products that will work.