Thanks @pandera999, @wanttotree.

OK, I think I understand what I need to do.  @wanttotree, Is my latest attached diagram here correct?

From @wanttotree's description,  I understand both routers (ie, the USG and the TP-Link AC1200) will need to establish their own PPPoE connection to MAXIS.

QUESTION:  Does Maxis allow 2 simultaneous PPPoE logins from the same account ?

@gaara92  @quasi @wanttotree

I have read that there are no formal specifications for the behaviour of "unmanaged L2 switches that are VLAN un-aware" when they encounter tagged VLAN frames.  Apparently some switches will just drop them whilst others will faithfully pass them along. (I guess others could just collide and keep trying ?)

OK ...curiousity got the better of me and I just tried the following setup to see if this particular L2 switch I happen to have (D-LINK DGS-1008MP) will just "pass thru'" the tagged frames or drop them.

As per diagram (sorry... rough quick draw) , I connected my wired PC to the ER-X and my iPhone to the WIFI on the TP-Link AC1200.

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Test 1:  with both the USG and the TP-Link AC1200 Routers ON

Ran Maxis Speed test on my wired PC (5 times) ....averaged about 91Mbps Download Speed
I have tried ringing the analog phone whilst the test I running too. Phone seem to work ok.
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Test 2:  with the USG powered ON and the TP-Link AC1200 OFF

Ran Maxis Speed test on my wired PC (5 times) ....averaged about 94Mbps Download Speed
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Test 3: with both the USG and the TP-Link AC1200 Routers ON

Ran 2 you-tube videos on my PC and 1 you-tube video on my iPhone 6. 
Ran all 3 videos streaming for about 30 minutes, no gliches.
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Not an exhaustive test I know, but it does look like a viable setup if one already has a spare L2 switch
and do not wish to spend more for a smart switch.  (For my setup, I am buying a TL-SG105e cos I
want my setup to be VLAN aware and cannot afford to depend on an L2 switch that happen to be well behaved).

Anyone can teach me how to bridge Maxis VoIP VLAN822 in edgerouter 6p?

Eth0 = WAN port
Eth1 = To internet clients (WAP, PC, etc)
Eth2 = To on-premise PBX .. this PBX need to see VLAN 822 tag to dial up the SIP account