It might be possible, but as @anav already wrote, it mostly depends on what D-link allows you to do and how exactly ISP delivers internet to you.
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Such setup would represent a possible attack path (ISP has their own way of accessing and managing the gadget and there wouldn't exist FW between gadget and MT anymore). And this is all possible because of PPPoE which in this case transparently passes the ISP router and ISP router doesn't have its own WAN IP.
The router/modem is actually mine and is not provided by the ISP, I just plug in the user name and password provided and it works fine. I really don't think there are any limitations from the ISP side.
Aside from the modem's brand, what I'm specifically asking about is the general use-case where common router-modem combos could be configured to pass all their lan and wlan traffic to the Routerboard connected down the network and then the traffic would be forwarded to the wan port on the same router-modem after passing through the RB. The clients connected to the build-in AP of the router modem would see the RB as the internet gateway. Most router-modems provide at least some static routing functionality. I wanted to know if, from a topological perspective using static routes (or some other means) that this is a valid case or not.
I'll attempt to draw the diagram in ascii to clarify my objective (sorry for my crude ascii art skills :) :
(Internet).........x..no direct route.x...............(Home clients)
___|____________________________________\/______ ........................______________
| PTM0(WAN interface)|..............................| LAN interface..|..Static Route?...| RB 750gr3........|
|...................................|<-Modem||Router->| (eth1-4 - wlan)|<----------------->|...........................|
|_________________ |_______________|____________| .........................|_____________|
................ /\________________________________________________________/\
(Dots are used instead of spaces because somehow multiple spaces are treated as one space on this forum.)
Most of Mikrotik's hardware don't have a modem built-in and have to rely on a modem from an external vendor, and it's very common for those modems to have wifi and extra ethernet ports as well as static routing. If what I'm proposing could be done, at the very least the router modem capabilities could be put to use as an extra AP and it could be managed by the robust RouterOS features.
The reason I ask here and not anywhere else is that I could be off-mark with using static routes and somehow RouterOS has a feature that could more elegantly fulfill what I am trying to do. As I said in the post, I'm an absolute beginner here!