I am confused you have two routers attached to the ISP modem with the same LAN addresses??
Or are you saying they have two separate WANIP addresses (from the ISPs LAN structure)??
Because the TPLink is dumb router with fixed settings, the MT is a smart router than expects the human to make the necessary settings.
I think its because the MT sees the external WANIP request coming from the outside(tplink) and routes the traffic back as appropriate for dstnat.
Whereas the MT request to the WANIP is internal and will not work due to this diagram.
See diagram Number 3…
https://help.mikrotik.com/docs/pages/viewpage.action?pageId=3211299
So you need to config hairpin NAT or loopback.
How check out this thread…
http://forum.mikrotik.com/t/port-forwarding-nat/140845/2
In your case, you only need to add the masquerade rule in the above thread as the dst-address=fixedwanipofthemikrotik