I have a Draytek Vigor 2762 which has been working well with my block of 8 static IPv4 addresses for a few years, but now my IP range is changing so I needed to update the router settings. It's still a /29 but different addresses.
When I first got the router it was easy to click on IP Routed Subnet on the dashboard and put in my range, but found out that the settings for IP Routed Subnet got removed in firmware 3.9.0. So my options were to downgrade to 3.8.9.2 again, or try and find out how to do it from the telnet command line. No idea why they would remove the options from the web interface when the feature was still working.
Google searches and the Draytek CLI docs didn't come up with any commands which mention setting the routed subnet, but after playing around I found them. The are sub commands of the ip command (which makes sense :) ).

ip pubaddr - set the public address of your router, this would be the same as what your ISP assigns to your router via PPP automatically, or at least what your ISP expects it to be.
ip pubmask - the netmask for your IP range, my my case it's a /29 so that is 255.255.255.248.
ip pubsubnet - enable or disable the routed subnet.

With all 3 of these commands you can give the ? option to see the current setting.
After setting a new address and/or mask you need to restart the router for it to become active and to show up in ip route status

Assuming my ISP gave me the range 22.104.145.10 - 22.104.145.17, and expected my router to be 22.104.145.16, I would run the following commands to set up routing:

ip pubaddr 22.104.145.16
ip pubmask 255.255.255.248
ip pubsubnet Enable

The PPP settings I configured normally using the web interface, they didn't decide to remove those options too thankfully :)

I then just need to assign the remaining IP addresses to the PCs on my local network. I did this by setting them statically, but the Draytek DHCP server does apparently support IP Routed too. The commands for that might be:

srv dhcp status
srv dhcp public ?

I never tried it though.