Your main router is EOL and the latest firmware is from last month.
You're okay for now, but you have a small window to upgrade your hardware to supported status. Then you can use the RT-AC88U as a node if needed (AX-class hardware is far superior to AC-class).
The link below shows what you may experience in your network too when upgrading to AX-class hardware. Note that the AC-class RT-AC86U was the previous 'king' of routers, and the RT-AX68U is an entry-level AX-class router (sadly, EOL today). And a single AX router simply dominates 2x AC routers... the king is dead, long live the new king...
There are some odd regressions being discussed on the dnsmasq-discuss list from the PiHole guy (Dominik). https://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2021q3/015598.html https://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2021q3/015596.html I see Merlin is testing 2.86 out...
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