This device only extends the reception area for DECT cordless phones. It will not solve your Internet connection problems. For that you need a WiFi repeater. For telephony, however, it is compatible with all DECT-compliant devices.
I only use Gigaset phones and with these the repeater works perfectly.
According to the operating instructions, "most DECT phones" are compatible with the repeater - but with reduced features: no encryption, no broadband connection (e.g. VoIP), no Eco
I have two S850 HXs and it works perfectly with them. However, I cannot answer whether it is compatible with the Sunrise Box, as I have only tested the following boxes (successfully): Swisscom Box and Fritzbox 7490.
Honestly: the Gigaset Repeater 2 is a joke product. The repeater function is completely inadequate and the range with the repeater is no better than without the device. Even at short distances, the network extension does not work adequately, which is why I advise against any purchase. A call to support ended in the recommendation to change the batteries of the mobile phones - complete nonsense. I also have a lift between the devices - but the fact that the network extension does not work is not due to the lift but to the miserable quality of the device.