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.
1. The WAN is intended to connect to the outside world, e.g. the Swisscom network. You may be able to configure it to work as a LAN, but in that case you would have to use the SFP for the network. Another solution, if you don't have a wired network, would be to use the 10GB SFP+ LAN port with an SFP-RJ45 module like this one:
Ubiquiti UF-RJ45-10G (check compatibility with router before buying)
2. Unifi APs can work in mesh, but with a loss of throughput. I haven't tested it with the U7 Pro but it works with the older one, so it will certainly work. It's very easy to install.
