- SQM doesn't work with software acceleration enabled in Firewall
- 500/500 is too much for SQM to handle on this hardware
This experimental firmware with NSS offload, has hardware-accelerated SQM via fqcodel: Ipq806x NSS build (Netgear R7800 / TP-Link C2600 / Linksys EA8500)
It may work for your 500/500 connection. You will have to disable software acceleration regardless (that build requires it to be OFF).
The SQM UI does not work for that build, so you'll have to figure out how to configure hardware-accel version of fqcodel via config files.