Hi Jeanloa,
why not use 2 x MR76s to create the MESH link..as you have dedicated antenna posts for both 2.4 & 5GHz. You could, for example, use the 2.4GHz radio on each AP to create the MESH link using patch antennas. This leaves the 5GHz antenna posts to provide local WiFI services. If the coverage isn't sufficient inside the building, then you could use longer antenna cables to locate the 5GHz antennas inside the building.