Solved. Turns out current PPPoE module was creating multiple PPPoE connections after one another without properly terminating the previous session and my ISP doesn't allow multiple PPPoE connections simultaneously. Hence it was taking time until ISP server resets my session ID>to connect internet.
So, yesterday I made a custom build firmware to add tcpdump-mini module. Flashed it filled in username & password for PPPoE connection. In PuTTY command propmt I ran this code
tcpdump -i eth0 -A pppoed
multiple times to gather PPPoE-discovery data until it connected to internet. Got 3 important fields from it,
- Access Concentrator
- Service Name
- Host-Uniq tag content (It is used by host to uniquely associate an Access Concentrator response (PADO or PADS) to a particular Host request (PADI or PADR))
Added those values in PPPoE-WAN interface along with
LCP echo failure threshold=2
LCP echo interval =5
Saved & Applied. Now re-connection is instant. If anyone has anything to add in this please do so. I'll keep testing for few more days then I'll mark this answer as a solution. 