I struggled with setting up this otherwise good router correctly.
I had several issues like speed drops, random disconnects, where rebooting or resetting the routher, or sometimes disconnecting and reconnecting the PPPoE
was required to get the internet back. Even after a reboot, it took about 1hr 50 mins to loose the connection again.
Now I have it running for more than a week without touching anything, without any problems.

Here's what I did (though I'm not sure which one is required for what :D )
Having the WAN configured correctly is not an issue, I have PPPoE with user/pass. Instead of automatic DNS, I've set up ISPs servers explicitly (Network - Advanced)
Same thing for IPv6 with ISP's IPv6 DNS servers.

When the router disconnected, it did not reconnect automatically, no matter what setting I used (Automatic or On demand),
so I rather chose to set 5 secs for Detect Online Interval in both the Network - Advanced and IPv6 Setup - Advanced tab. (default is 0)
No disconnection issues so far.

for PPPoEv6 I use shared session, DHCPv6 for addressing type and SLAAC for LAN.

With IPv6 I alsohad speed issues, mostly on wireless.Speed dropped to as low as 0,1Mbit/s which is quite bad on a 500Mbit connection.
The solution is quite simple though: enabled Bandwidth control and set the "limits" to the max allowed (1000000) and set an IP range with the same rule.
Wired and wireless here are acting differently, so I had to exlude my wired IP from the IP range in order to get the full speed provided by my ISP (500Mbit/s)
Here the strange thing is that the "limit" is actually 2 times higher than my WAN speed. :o It looks like the function itself is required for IPv6 to work correctly.

I've set up the 2.4GHz to use 11bgn mixed (default), but to 40Mhz and channel 8 (this seems to work the best for me),
For most of the devices, the difference in speed is HUGE. 90-95Mbit/s on 40Mhz compared to 35Mbit/s what I had on 20MHz.
For 5GHz I left defaults alone

I have an Archer T2U (the only device connecting on 5GHz). It also had some speed issues.
After removing the printer (as the only obstacle), though the adapter is pretty close (120cm) to the router, the speed was still below what I expected,
Then I came up with the idea of adding a MAC filter for the adapter. And it seems to be working :)
I dont't really understand why, but is now finally able to hit its theoretical max (~210Mbit/s)

For wireless I'm using different SSIDs for 2.4 and 5GHz, I only changed the password for wireless security.
No guest networks set up.

ps: I also have a couple of URLs blocked, some IP/MAC bindgings, access rules, but these were always working without affecting speed or stabilty.
I hope it helps :)