I'm also a newcomer to Ubiquiti land (proud owner of a UDM SE and a U7 Pro since this week).
First of all: the U7-Pro has no power supply. This means it requires power via PoE.
When I look at the tech specs of the Dream Machine Pro Max here,
https://eu.store.ui.com/eu..., neither the 2.5Gbps port 9 nor any other port has PoE. While the Dream Machine Special Edition has two GbE PoE ports, the Dream Machine Pro Max doesn't seem to have a single PoE port on board.
However, the 2.5Gbit WAN port can be converted into a LAN port (theoretically, not tested but researched online). To do this, you have to map one of the eight LAN ports on the left side to port 9 (the 2.5Gbit WAN port) in the UniFi OS settings. See this picture, directly from the settings of my UDM SE:
https://share.cleanshot.com/vjMBqLfrHere are two more links:
https://www.reddit.com/r... and
https://www.reddit.com/r...So in THEORY this should work. However, I have not seen confirmation anywhere that the U7 Pro is connected to port 9 and successfully acts as an AP for the local network.
However, port 9 does not have PoE either. So you would also have to buy a PoE+ injector to supply the U7 Pro with power (pay attention to the wattage).
The solution I have opted for here is to buy a multi-gig switch with a few PoE+ ports. Depending on the model, you can then connect it directly to one of the SFP+ ports of the Dream Machine or to port 9 or one of ports 1-8 (the speed then changes, of course, depending on which port is selected). I get this one myself:
Ubiquiti Enterprise 8 PoE (10 ports) and a "Direct Attach" cable. This gives the internal network a full 10Gbit from the second SFP+ port on the Dream Machine. (the internet connection is connected to the first one). I then simply connect the U7 Pro to one of the switch's 2.5Gbit PoE+ ports.
Last but not least... apparently the answer to your mesh question is: YES. All Unifi access points are mesh-capable.
However, if you want to expand your WLAN and don't have the option of pulling additional network cables, you can't use a second U7 Pro (it needs PoE+ and doesn't have a power supply). So you would have to rely on other APs without PoE. But at least that should work well.