I haven't done RTI in a very long time. I can't remember if the XP6 could be set with a password or not. That would be the only thing that could get you, is if the previous installer set a password on the XP6 (if that was possible). Then you would have to reset the XP6 and program from scratch. That is not something a normal consumer can do without a lot of instruction and experience (especially since you said you have lighting & HVAC controlled as well).
But if it has no password, then yes you can connect to it with the ID software and see how everything is configured.

Since the AD-8x uses UPnP with the XP6, then technically it shouldn't matter the IP address. And maybe Diskreet's advice of just keep rebooting it until it connects again, will eventually work. But it seems to me, if it hadn't done it after 2-3 times then something's wrong.
Might also check and make sure UPnP is turned on in the router. A lot of routers these days are coming with UPnP turned off by default. That would make this not work.
As far as static IP goes.... You should still set up static IPs for ANY equipment that is a permanent part of the network and on 24/7. Bonus points for doing VLANs with a managed switch and separating out streams, IoT, control, etc. for a more efficient & secure network. And document everything! That's just good practice regardless of the stupid $hit manufactures sometimes say.
Sometimes when dealing with crappy manufacture that their product just barely works on a network, you might have to follow advice to not set a static IP in the device itself - but you should still set a DHCP reservation to make it static on the network.

[edit] If you don't already know - a LOT of AV manufacture (even the really good ones) struggle sometimes with network stuff. They just don't have the talent in-house to have great networking results. A lot rely on 3rd party design for their network interfaces with firmware/software. Many examples thru the years of great products with crappy & troublesome network issues.