Thanks for the quick response. I have already been able to setup (via LAN) Archer ZR1600v is the primary modem/router and the ASUS DSL AC68U is the secondary router (mode) as a slave.
I need to bridge them via WiFi so I can use the ASUS at the other end of the house where the Solax Inverter is. I would like to know how to setup both for the WiFi bridging.
The WDS or WiFi bridging is not possible with your TP-Link VR1600.
This means that you are not able to connect them via Wireless.
You may consider a Mesh network or a longer cable to relocate your ASUS router near the Solax inverter.
In case anyone comes across this post, I managed to get pfSense working. The solution is to not over complicate things! If the modem is bridged using VLAN ID 2 then there doesn't seem to be any need to also set up any VLAN on pfSense. I simply put in my username/password into the PPPoE Configuration of the WAN interface and it worked. I haven't tested whether DNS is an issue, but I did check the box in General Setup for "Allow DNS server list to be overridden by DHCP/PPP on WAN" but that is all.