While MW3's UPnP compatibility with various routers is indeed spotty at best, the question does not contain enough details about the router settings to provide an authoritative answer.
Based on my best guess from what little information provided, as it seems unlikely for the UPnP to treat the two connections differently in and of itself, it would seem that you have different settings for one of your connections than the other.
This could be the case if you setup a static IP and/or port forwarding for your wireless adapter, but did not do so for your wired adapter. Your wireless and wired connections will have different physical addresses on the adapters and as such will be treated differently by your router. You will need to have the "same" settings for both in the router in order for them to be treated the same - obviously they could/should/would have different static IPs, but both simply having the static IPs and appropriate port forwarding to their respective IPs should suffice.
If you feel certain that UPnP is responsible or related to your issues, there are some UPnP testing tools that you can try at your own discretion to ensure that your UPnP settings are correct. Further information about your setup would be helpful in diagnosing your issue further if needed.