With reference to this topic. I did attempt to do the samething.
I was helping a friend to change her original Maxis router to TP-Link AX5400 which is more powerful. At the same time, I figured out that the phone cannot be used.
1. I asked my friend to call Maxis CS if we can connect the phone to ONU instead of the Maxis router. She (from Maxis CS) said cannot. I asked for the ONU login, she didn't give what I wanted. She just repeat the PPOE username and password.
The Maxis CS said I can only connect my own custom router after Maxis router eventhough I told her I am very confident that I can use the phone ports in ONU.
She also don't want to give the SIP information. She said she don't know what is SIP.
2. Hence, I asked Maxis CS to give the administrator login for Maxis router. She gave and I found out the SIP details.
3. I found out the login for ONU which is HG8240. I able to create VLAN 822 and obtain IP from DHCP.
4. I able to configure VOIP on POTS 1 using the configuration I copied from old Maxis router. Phone is working.
Lesson: If you are confident with your technical background, go for it and don't believe everything from the CS.
You are lucky to have a HG8240 without the webUI disabled.