Hope someone here has a solution for my problem

I have an Asus AC66 router connected to TM modem. From the Asus LAN port 3, i then connect to Maxis Aminia MA131 WAN port in order to use their VOIP.

I tried taking out their 4g sim card from the Aminia router because wanted to put the sim into a USB dongle to be pluck into the Asus router as standby dual WAN. However when the sim card is removed from maxis router, my VOIP stops working.

Is there a way to configure maxis router in order for the VOIP to work without needing the 4g sim card?

Where to find the vlan id in Asus router settings?
I can only see the vlan setting in the Aminia settings as shown in picture.

TM modem model Huawei Echolife HG8240H.

Yes I'm aware how to set the iptv to maxis fibre mode. alfredhow78 was asking me what vlan id in my lan port 3 which i can't find anywhere in my asus router settings. Oh well.... i guess it's 621 for tm unifi backbone.

My problem now is how to get the voip working without needing the 4g sim to be inside the aminia router.