I reckon it'll work and to just buy one for 60eur off ebay-klein and try installing. Mr Google fund ...
1 Unlocking your FRITZ!Box
If you purchased your FRITZ!Box from a retailer, your cable provider must unlock it first:
Important:In Germany, you can only use the FRITZ!Box 6490 Cable with the article number 20002778 with all cable connections. Models with other AVM article numbers are not intended for use with all cable connections.
Have your cable provider unlock your FRITZ!Box. Depending on the provider, you can either use an activation portal or call their customer service to activate it.
With some providers, the serial number and the FRITZ!Box's "CM MAC" address are required to unlock it. You can find both of these on the bottom of the device: