The GT-AX11000 needs rebooting so you want to replace the RT-AC86U? ???
As you don't specify what troubleshooting you have actually performed or described what the symptoms are, throwing more money at the problem isn't the best course of action right now. Even if it will be proven to be at the end.
Does the GT-AX11000 show stability when it is the only router in the network?
Have you tested different Ethernet cables? Are there switches or other network infrastructure that may be affecting things?
What specific troubleshooting steps have you performed?
What specific issues are you facing and what logs, indicators or other symptoms does the network display when you need to reboot the main router?
How are you adding the RT-AC86U as an AiMesh node?
What firmware are the routers on, now?
I would suggest first flashing the RMerlin firmware on both, following the WPS Button reset method applicable for each as shown in the link below.
[Wireless] ASUS router Hard Factory Reset | Official Support | ASUS Global
Then, get the GT-AX11000 router stable as the main router first, before adding the RT-AC86U as an AiMesh mode as described below.
Best Practice Update/Setup Router/AiMesh Node(s) 2021
If issues continue to persist, you may want to fully reset both routers you have as suggested by following the steps below.
Fully Reset Router and Network
And afterward, follow the best practice link above once more.
If the steps above do not bring your network/router stability, you may want to give further information and specifics of your network to get better advice.
Or you may find that one or both are suffering from (possible) hardware issues too.
Either way, following the steps above, will ensure that configuration/firmware interaction issues are eliminated as much as possible and you are actually trying to troubleshoot solvable issues instead.