Your router doesn't support mesh (specifically, AiMesh).
Is your goal to extend the Guest network around your home?
You will need to buy at least two RT-AC68U's. But, not recommended today. Too old, slow, and with limited range and throughput which all affect stability and reliability too.
Where the game starts is with two RT-AX58U's. But again, not recommended (for the price they currently command).
Two RT-AX86U's is where the benefits of upgrading will show. Particularly if you can have the AiMesh node, wired. On sale, with any luck, you will find them at or less than the RT-AX58U (which they run circles around).
How big is your home? How many levels? How many concurrent client devices are in use at any one time? What ISP speeds do you pay for?
I would guess (without knowing any of the above) that a single RT-AX86U will easily surpass what you're currently using. Without needing a second unit for a 'mesh' at all.