It is true, our SM only connects to 2.4g wifi networks. However, the Sync Module should work fine on Dual band and mesh RTR. You may need to do a few things on the RTR.
There are a number of issues with earlier (8ish months ago) Asus RTRs and the 2.4g channel dropping or not accepting new devices. Make sure you have the latest Firmware from Asus installed on your RTR, turn off any parental controls and then try it again.
Netgear is having a similar problem right now on the Nighthawk 7K/8K models there is a lot of conversation on their forums about the 2.4g channel falling off and or denying additional devices.
If you do the above and still have a issue, call the support line, you can simply press the Call Tech Support button in the Blink app.