Okay, after spending about 4 hours, I think I finally set it up. Don't have step-by-step instructions because a few things went wrong and I had to mess with a number of settings, but the basic idea is this:

- Connect Archer's Internet port to Pace's Ethernet port.
- In Pace's Firewalls etc configuration, select Archer and set it to the DMZPlus mode.
- Connect Archer's Ethernet port to your switch (if you have one).
- Turn off Wi-Fi settings in pace's configuration (for all frequencies and guest networks as well).
- Reboot Pace and Archer.
- Go to Archer's web admin page and use quick set up (make sure it use DHCP or Dynamic IP).
- Select the troubleshooting option from Pace's home page. Make sure everything is correct (mine complained about the router-behind-router condition, so I went over quick setup for Archer, rebooted it, reran troubleshooter and it all worked).

You may need to tun off your connected devices (PCs, WiFi appliances, phones, etc) while you're doing this (I had an issue with the Ecobee thermostat which could not reconnect, but once I rebooted everything, it worked fine; i assume it was an IP address conflict of some sort).

It works so much better now.

Thanks for your sharing, I will notice that if my C9_v3 faced the same.