Found this thread very useful - so just want to share my discovery in case it help others too. I had quite a different discovery from various postings here so kinda strange but it works.

I am on maxis fibre & voip, plus astro iptv, was using the standard alcatel TM modem + thompson router

Bought linksys wrt1900ac to replace the thompson, but i need voip, so still connect thompson to linksys just for voip (WAN of thompson to LAN 3 of linksys). Astro IPTV connecting to LAN 4 of linksys.

The linksys router has built in "maxis" vlan preset - specifying WAN 621 (tagged), LAN port 3 for voip 821 (untagged) and LAN port 4 for IPTV 822 (untagged). Tried this preset - VOIP not working, IPTV works but could not access Astro's On Demand function

Have tried changing to manual vlan - 822 for VOIP LAN 3 &  823 for IPTV LAN 4 (all UNTAGGED) - same issue
Have tried changing to manual vlan - 822 for VOIP LAN 3 (UNTAGGED) - no VLAN tagging for LAN 4 - VOIP still not working, but IPTV (including on demand) works perfectly!

So it seems Astro IPTV does not need vlan tagging to work - if vlan 823 is setup, on demand could not be accessed. Don't know why, but since it is working perfectly - then it is what it is

For VOIP, after trying so many variations (no vlan, vlan 821, 822) it finally works ONLY after i changed VLAN setting of VLAN 822 to TAGGED. From most of the posts here by right the VLAN 822 setting should be UNTAGGED, so again it's kinda strange mine only work if TAGGED.

Anyway, after 3 days of googling and scrambling through this thread and others at least it is working as i intended now. So just sharing in case it helps others - because i understand how frustrating (but somewhat exciting!) journey it is to get rid of this lousy Maxis stock router.