I'm using AX86U with 2 other 5-series Asus routers as AiMesh nodes

House is 3 levels ~5,000 sq ft with no issues

Try to use only the 20Mhz channel width for 2.4G and 40Mhz for 5G, which will give you the best balance of speed / stability

If you would like to use the 80Mhz or 160 Mhz channel width for 5G, you will likely need to to setup some mesh nodes

Also, use the upper channel numbers for both 2.4 and 5 bands