Those voice ports (Phone 1 & 2) can only be used with Optus' fixed-line phone service. So if you have an Optus home phone service Phone 1 should automatically be enabled.
YetanotherAcc is correct, @Brendanandjen. You can learn more about your plan inclusions via the CIS of your plan. We also have Modem Guides to help you set up your landline if you currently have a landline service configured on your account. If you would like further assistance, I would recommend engaging with our Technical Team via our 24/7 Messaging Service on the My Optus App
Has the phone ever worked with the 5366LTE?
Do you have Optus Cable or Optus NBN HFC?
I assume your internet is working at full speed, correct? When the 5366LTE is in 4G backup mode the home phone will not work and your internet speed gets capped to 12Mbps. This should not be your problem though as 4G backup will only run for 30 consecutive days and you say you have been with out phone for months.