In principle, this is possible, but it requires a few settings (if possible in the Sunrise Box). Therefore, the question would be whether there is access to the Sunrise Box in order to make the UDM connected to it an 'exposed host', for example. If this is possible, it works, although the combination is not ideal due to the double NAT. Shares, if they are to be accessible from the Internet, must then always be entered in the UDM and the Sunrise Box. The best thing to do is to search for 'UDM behind Fritz!Box', which is a different (popular) router in Germany, but then you will quickly get a few hits and can read up on the subject. Then you have to see to what extent this can be transferred to the Sunrise Box.