I know, that this is quiet older topic, but I found it, because I have similar problem. We have at work OKI MB441n printer, and with older router (TP-link TL-WR740N) it works fine. I have bought MikroTik RB751G-2HnD (OS v6.13) for better performance in our local network.
I was very dissapointed, when I figured out, that our printer cannot get correct IP adress from DHCP server, after many attempts, I get the printer to the local service center for upgrading it's firmware, because it was not feasible at home/office for our own. Upgrading costs about $30 and after that, i hoped, that it will work fine, BUT it was NOT! The last chance, was advise from service technician, he said: "Try to switch your ethernet port from Gbit to 100Mb, because older OKI printers have problems with Gbit ethernet lan version." SO I tried put this port into 100Mb full/half, and get again nothing
When I saw, that there is an option to switch the port to 10Mb speed, i tried THAT and, my friends it works!!! After two weeks of many attempts of configurations about DHCP etc., it was this problem!
Conclusion: Older OKI printers (we have 2012) have troubles with full Gbit speed ports, so try to switch them into 100/10 speed and it should works fine! It's a shame, that manufacturer haven't made a firmware update for fixing this problem.
I hope it will help someone dealing with similar troubles.
PS: I wondered about returning MikroTik to seller, thinking it's the bad one, but it wasn't.