[Dual WAN] One computer to only use one wan

[L3ctric]

Hi,

A simple question with perhaps an advance answer.
I am gonna get a second wan. Though this is a 4G connection. I have 200gb there and i will use load balancing.
Though i do heavy downloads for games, movies etc.... So I want to stick with my ADSL. Thus, I bought an Asus router(ac-87u) for dual wan.
However, I want some computers only to use the ADSL wan while others use both. Then i can keep downloading large stuff on some computers while others use the high-speed 4g network.

This maybe not possible with standard firmware but is it possible with say merlin or dd-wrt?

Thanks in advance.