RV340W - IPv6 Prefix Delegation nonfunctional

[train_wreck]

Hello. Very simple, I am trying to enable IPv6 prefix delegation on a RV340W. Device has the latest firmware, version 1.0.01.18. The previous router at this location (Cisco RV042G) had no problem doing this with the exact same setup/ISP, but the customer specifically requested an upgrade to the RV340W. The ISP is AT&T, and the modem is a Pace 5268AC.

I have enabled prefix delegation on the WAN page (WAN1 in this case):

 

And on the "Status and Statistics" page, there is definitely an IPv6 address shown:

But I cannot assign an IPv6 address to my internal VLAN (I am intending for VLAN20 to have the delegated /64 assigned to it):

All it says is "unavailable". There is absolutely nothing in the logs indicating any kind of failure (or success, or anything really. Pretty useless IMO).

So what is the problem? Is IPv6 prefix delegation broken on this device? I'll point out that the RV042G is over 10 years old and handles this with no problem.......

[Mike Johnson]

Check that IPV6 is enabled on the Pace modem.  ATT has a new modem that replaces the Pace 5268AC with faster boot-up and more security features, but it still lacks a bridge mode and instead use IP pass-through.  I would suggest calling ATT and tell them you are having modem problems and would like to replace it with the latest box at this time is the BGW210-700.