Hey there, I have added 6 older Engenius boards to the project, 4 of them are still pending review. This is a much newer one so there's probably a lot of differences.
First, open the board, remove shields if there are any (like a tin lid), record the model of every chip (SOC, WLAN, RAM, FLASH, external ethernet switch if there is one).
Never assume hardware will be the same as another model.
(btw the people in the mailing list were mistaken, IPQ chips are Qualcomm not Broadcom)
Here's a general list of the other things you need to get started
FCC ID (I am guessing this is the device)
(unfortunately the FCC photos for this one are really bad)
kernel log (dmesg or copy serial output)
(only the serial console output can tell you which partition is being booted)
(either printenv from uboot or fw_printenv after booted)
dump of the ART partition hexdump -C /dev/mtd7
GPIO numbers and ethernet PLL registers
(PLL registers need more explaining later, and I don't know how it works for IPQ SOCs)