Already confirmed for you in the post above.
AC88U + AC86U/AC68U for AiMesh
I have an ADSL modem coming into a very small panel near the entrance to my home. The panel is big enough to house a 68U or maybe 86U. Inside the panel there are 3 wired ethernet connections from whatever router I put in the panel to each room in the house. What I am planning to do is shove the...www.snbforums.com
You may be asking for trouble making an AX + AC router an AiMesh network though. The RT-AX86U is better paired with an RT-AX58U or better. That much older RT-AC88U may be a source for bottlenecks and/or other issues too when paired to an AX main router.
Your goal for either an AP or AiMesh network should be to have the same class of Wi-Fi (AC, AX, AXE, etc.) for all your wireless routers. Not only will this give the best speed, lowest latency, and hopefully the best seamless roaming for your client devices, but it should help too with getting a solid, stable, and reliable network too.
(Also, what if I turn off the wifi on the AC66U+?)