how to install domoticz on ZyXEL NSA325 v2

You can install Debian. But maybe you'll have to compile Domotics yourself. The pre-build images contains only an Armv7l  package, your '325 runs an Armv5.

Don't know much about Domotics, in their forum searching for Armv5 I found some questions of people running a Synology Armv5, who had the problem that no recent nodejs is available. If you need nodejs, you're out of luck. Armv5 as target for nodejs is dropped years ago, so now only a very outdated version is available. (v0.9? Now we have v16 or v16)