I have two ISP's, one primary and the other for backup.  When I connect to a certain wifi SSID, I want to connect to the backup connection.  I am pretty proficient on simple networking but not good on VLANs, but I am thinking I could use a VLAN for this.  

I use a couple of Netgear R7900 series as AP's but they are not smart enough for VLAN's.

I have an ER605 router and a SG2008P switch.  I see where they both can do VLAN's and since they are the same maker, I hope it might be simple.

My thinking is to assign a VLAN to the WAN that I want (I am using 66) on the ER605:

I then assign that VLAN (66) to a port on the SG2008P (port 6):

Next I attach an AP to that port (6) on the switch.  Would any traffic coming in on that AP then be assigned VLAN6 and route to the desired WLAN? 

Also what is tagged vs untagged.  Since my ER605 says 'TAG' should I make it tagged on the switch?

Thanks for any help or direction to a place to learn all this.

Roger