Maxis had done the dirty tricks.

They removed the "BRIDGE HSI(GPON) + VoIP" setup option in order to take control (or perhaps may monitor?) your connection to internet by go through their censored DNS server.

I just realized that they can modify the Setup Templates to remove this "Option" whereby it was online when you configure it.

They did a dirty trick by breaking your VoIP connection, then at the time you inspect the connection by test out to re-setup the Thomson Router configuration then you dropped into their trap.
Permanently removed "BRIDGE HSI(GPON) + VoIP" setup. And it should be said that they had restricted their user to use the BRIDGE Mode for those subscribers.

I still figure out how to configure the settings by editing the user.ini configuration file that exported from Thomson Router.
But by my knowldedge on .ini configuration and limited source from Thomson, it is quite a tough job for me for understanding unfortunately. And not much time to learn for.
But I think to edit the Configuration File then stored back to the Thomson router, it could go for BRIDGE Mode I think.
(*Maybe someday)

Currently the best way to get VoIP connected whereby abandoned Maxis Thomson Router is to modify the Huawei HG850a BTU. Unfortunately mine is the old Alcatel I240-P BTU.
I have got a piece Huawei HG850a BTU but I didn't have the GPON settings on this BTU, so I can't get it on Maxis Fibre. I have seek those who really willing to share their GPON setting file for me, but sadly no one is helping.
Perhaps they worried someone to intrude their Ethernet.
Finally I can't have my Huawei HG850A BTU works on the GPON.

For your question, since the current Archer C7 firmware does really supports Maxis Fibre, it is best to stick on the stock firmware than OpenWRT or DD-WRT because of its AC1750 WiFi.
OpenWRT/DD-WRT will make you loss the AC1750 WiFi function as Atheos AC1750 driver is not open for public.
(Of course you would have a better control & management on your Ethernet by OpenWRT/DD-WRT)

The 2nd question about the subnet.
You can configure Maxis Thomson and Archer C7 in the same subnet, but need to know which is the Host to connect the Internet (Dialer).
Currently you can only manage Maxis Thomson to connect to internet in order to have their VoIP supports at the same time, and Archer C7 to be your sub-Ethernet device from Maxis Thomson.

Of course by this setup, your Archer C7 WAN port connected to Maxis Thomson LAN port.
Setting up the Archer C7 "WAN" setting to "Dynamic" or "Static" (<- "Static" to fix IP as a connected device for Maxis Thomson under same Thomson subnet).

Then Archer C7 "LAN" change to different subnet at this point, this subnet managed to be Ethernet within your devices connected to the Archer C7 but not Maxis Thomson
.
Else if you are using same subnet under Maxis Thomson 192.168.1.x, which Maxis Thomson using 192.168.1.254 as a Gateway, even you connect your devices to Archer C7, those devices will stick on Archer C7 192.168.1.1 as a Gateway, your devices will not get the Internet connection since 192.168.1.1 is not the device to have the internet. Unless every device you need to configure their Gateway to 192.168.1.254.

If you change the Archer C7 "LAN" setting to different subnet, said 192.168.0.1, then all the devices connected to Archer C7 will be within 192.168.0.x subnet where Archer C7 (WAN) host the internet from Maxis Thomson 192.168.1.254 (LAN), those devices connected to Archer C7 will not need to change their Gateway for that extra jobs, it automatically sticks on 192.168.0.1.
Lastly, I have bought a Linksys PAP2T-NA VoIP Adapter which not expensive, I am going to make it works as a VoIP device by connecting Archer C7, but currently I don't have the time to test it out. Maybe Sunday.

Else if someone really willing to share their Huawei HG850A Configuration file which I am really thankful for that , then I will no longer need the VoIP Adapter, only connecting VoIP by HG850A. 

My aim is to totally eliminate the Maxis Thomson Craps.