If you have this modem then it sounds like your NBN connection is not active or is down and you are running off the 4G backup.
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.
Assuming that you have done your speed test via LAN and not wifi then there is not much else you can do except contact Optus directly and have them investigate.
If you have FTTC/FTTP/HFC the only other test you could do is to try and rule out a faulty modem (unlikely) and connect directly into the NBN box or use any other router.