Make a OpenVPN client have the same IP every time

I have 2 synos, one connected to the other via VPN, as the only client.

It used to be that the client would always get the same IP address, however after a VPN server update a few weeks ago, it now gets a new IP every time. This breaks HyperBackup, which expects to find the target at the same IP every time...

I've already tried to add

ifconfig-pool-persist ipp.txt

to

ifconfig-pool-persist ipp.txt

and restart VPNserver, however it doesn't seem to work...

Does anyone have any idea what's going on here?