Hey John, not sure if this is the best thread for my question, and I will move it to another if necessary, however considering it is with 3 routers on your fork, I figured you might be best for input.
Short version is that I had 2 routers and a media bridge; N66 broadcasting 2.4GHz, 20MHz on channel 11 to the EA-N66, which was wired to the AC66 on WAN to broadcast 2.4GHz, 40MHz on channel 1 above. This worked well to extend my range without overlapping channels for a couple of years. The one odd part is that the EA-N66 was on a different subnet in media bridge mode, so my extended network was as well. I jumped on a deal for the AC68P and opted to "improve" my setup by replacing the EA-N66 in the equation with the AC66 taking it's place (figuring stronger antennas for TX/RX between it and the N66).
Now I have the N66 (router) broadcasting to the AC66 (media bridge), plugged into the AC68 (AP). I have used both 20E9 as well as 22B4 and now 22BA, but have an odd behavior. In this configuration, the AC66 does not operate on a separate subnet in comparison to the EA-N66, which did, and will instead either get it's IP from DHCP on the N66 or through a static assigned IP (what I have opted for as you will see). My problem is that after about 12 hours, clients can connect to the AC68 on either 2.4GHz or 5GHz, but are not able to get a DCHP IP from the upstream N66. The AC66 is still connected with good signal, but DHCP requests stop working for some reason. It appears that they are made and don't get to the N66, which is the only router with DHCP options as it is in router mode rather than AP or media bridge. I could put the EA-N66 back in place instead of the AC66, but would rather avoid doing so. Through testing, the only router that I have had to reboot to fix the issue is the AC66 (media bridge), at which point clients connecting to the AC68 get DHCP addresses again. One last tidbit, after changing the DHCP range on the N66 from 192.168.1.5-192.168.1.254 and setting the AC66 to 192.168.1.2 and AC68 to 192.168.1.3, when the issue occurs, if I connect to the N66 wireless, I can open the management page of the other routers. Also, if I set a static address of .4 for a client that is connected to the AC68, I can connect to all three as well, so it appears to clearly be DHCP related.