turbo QAM must be set to enabled (for both 2.4ghz and 5ghz)
For congestion control provider I strongly suggest using “bbr” which will then automatically turn into bbr2. Cubic is the second best choice in my opinion
TurboQAM is essential for 802.11ac and it cannot be disabled.
And TCP congestion control only applies to connections whose endpoint is the router.
For example, if you connect to the router's WebIF or even if the WAN connection type is PPP.
If the WAN connection type is DHCP, it does not do anything.
... or the last DDWRT builds have some wifi issues (those are in WIP atm)..
Isn't DDWRT always a work in progress? I mean all builds are beta these days, no stable releases.
Edit: I'm up to > 500 mbps down with the suggestions in this thread. Upload is ok ~300 mpbs. That is probably the best I can do with the wifi interference in my location.