need to establish another three tunnels between b, c,d.

Obviously the router just has public MIB OID, which always come with some snmp client. The OID is public and easy to get.

What is your hardware and firmware version?? Maybe you are faced with the issue below. TP-link FAQ is a useful document and some material to use: https://www.tp-link.com/en/faq-1029.html

soupraider wrote i've done that already but still dont work, but right now we change our isp to dynamic. how to set my device ? Have to configure on the Network--->WAN--->connection type, on the...

I have some TL-ER6102 V2, whose web interface is the same as yours. I'll have a try later.

They both 1 WAN port and 3 WAN/ LAN changeable ports, which means it has up to 4 WAN ports.