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Do you have this modem? If so, it may be running in 4G backup mode which has a maximum download speed of 12 mbps.
Q: How do I know when my Optus Ultra WiFi modem has switched to 4G back-up mode?
A: The Optus Ultra WiFi modem has a Network and Signal light indicator. When the modem is in 4G back up mode, the Network indicator will light up. The Signal indicator will display the strength of your 4G signal.