@tr6pilot,

I would recommend first making sure that your Guest Network is active and visible from other devices on the network. Decos have an additional setting that will stop broadcasting the guest network SSID after there are no devices connected to the network for a set time. 

As you are trying to repeat the 5GHz signal, make sure that your RE is not too far from the closest node. 5GHz signals are much more susceptible to wireless interference and the signal may not be strong enough at the location of the RE. Can the RE see the network when moved closer to the Decos?

Have you reconfigure the RE or were you trying to mimic the connection settings from before your upgrade? Has the RE been reset since changing the main router out?