That's frustrating, but at least it's working on the old version.

Contact TPLINK and advice them. It does appear they have left this modem out of their latest release?

Unfortunately, the ZTE AC2726 is not listed in the TPLINK modem .bin files?

Facing same problem with the latest firmware on TL-MR3220 V1. The modem gets recognized, but PPP authentication fails for some reason. The syslogs are not detailed or descriptive, so hard to tell what's going on.

One problem that I suspect is that using the ISP provided dialler, one has the option of choosing between CDMA 1x only mode, CDMA EVDO only mode and the hybrid mode. Not sure if the usbswitchmode utility is able to distinguish between them, and make the right settings before authentication, although, that in itself, I doubt could be the reason for the inability to authenticate.

This router otherwise, works fine with a Huawei E156 (GSM) modem, with Tata Docomo and Aitel 3G SIM, in India, and overall configuration is fine, without doubt. Have written to TP-Link support, and hope to receive some helpful answers soon.