Yep.. mobile as well and its equally as poor.
Speedtest a moment ago
on Mobile = 2.87MBPS
Broadband = 0.43MBPS
been on the phone to Optus past 3 hours and they are agreeing to send a new modem. They can not fix the issue. Speeds have been <1MBPS then up to 3MPBS for 5min then down.
Thats a great site. closest tower 1.8k away
But Optus tell me I am connected to a tower 6.5km away?
They have agreed to replace the modem
As Tigers1719 has suggested to the OP, after doing troubleshooting it's best you contact Optus directly. You may need a replacement modem.