I've been using AX86U router with AC86U as node for 6 months, perfectly fine! Just in the Wireless setting of 2.4Ghz, make sure do not leave it as dual bandwidth 20/40 but select either 20Mhz or 40Mhz and control channel set to either 1, 11 or 6 (this setting will be propagated onto any node's 2.4Ghz). I had AC68U as node before,,,it wasn't that good , that's why I am now using AC86U as node!
Why single bandwidth rather than 20/40, this was what I observed, while AC86U node 2.4Ghz had 20/40 bandwidth (mostly default settings on 2.4Ghz), the AC86U node's 2.4Ghz sometimes (after like a week or two) had denying connections, connected but no Internet for existing 2.4Ghz client, new 2.4Ghz could not connect to the node..etc HOWEVER the 5Ghz interface worked perfectly during the incidents!
I contacted Asus Support since AC86U had previous issue of 2.4Ghz radio dropping/denying connection in old batches such 2017-2019 (thats what I read/researched from all around) and said 2020 batch was fixed! (I do not look into details what Asus fixed in their AC86U prod batch but some said 2020 manufactured date was better and problems seemed to be fixed). Thus, Asus Support confirmed yes your AC86U 2020 batch,, please apply this change onto the AC86U, they told me to switch 2.4Ghz's bandwidth settings to single bandwidth and change the control channel within 1, 11 or 6.
I did apply the new settings for 2.4Ghz wireless, 20Mhz (previous I set to 40Mhz worked ok but read around the 2.4Ghz was designed to work best in 20Mhz bandwidth and compatible most Wifi client) and left my control channel to 1.
After a few days, I haven't had any buffering issue nor 2.4Ghz became inoperational ...select single bandwidth in 2.4Ghz seems to be improved (NOW I think I was having buffering issue, slowness onto the AC86U node while using Merlin firmware ,,,,,it must have been dual bandwidth setting in 2.4Ghz causing the problem!!! well maybe try Merlin fw again later!)
Good luck!