A /29 is mask 255.255.255.248, yes.
If you are configuring this as a routed netblock, you will lose 2 (actually 3) of the IP addresses:
- One becomes the network address (the lowest in the range).
- One becomes the broadcast address (the highest in the range).
- One will become the default gateway of the network (the IP address in that range you give the router).
Having said that, X.X.8.11/29 does not lie on a network boundary of a /29 network (so not sure what's going on there). Valid networks would be X.X.8.0/29, X.X.8.8/29, X.X.8.16/29 etc etc.
In the Draytek under LAN -> General Setup, there should be an option to enable IP Routing Usage where you specify a secondary IP address and netmask - this is where you put the details in. There could be an option to activate the second DHCP service too for this network (if required). This is based on a Vigor 2800 - for yours, it maybe marginally different.
Hope this helps.