That’s what Synology actually suggests: access your router as router.synology.com. Doesn’t that mean, it’s under control of Synology?! Looks like, doesn’t it?!! I don’t really like that.
It also has a hardwired 192.___ address (to be clarified!!!).
I am currently running my RT1900ac between my laptop and my true router, an AVM FRITZ!Box. The RT1900ac publishes its name as SynologyRouter, so I can also access my RT1900ac as SynologyRouter.fritz.box.
And certainly my AVM router also assigns a local IP address to the WAN port of my router, that I can look up in my AVM router’s administrational interface.
On the web interface at https://SynologyRouter.fritz.box log in as admin – it will ask you initially for a password.