QOS with n450

Model No: N450/CG3000Dv2

I currently have a N450 setup on my home network running fine. I also have a VOIP basestation plugged into it(Ooma). The problem is sometimes I have poor call quality: people complain they can't here certain words, call going in and out, etc. The most likely issue that comes to mind is not enough bandwith for the VOIP device. So I would like to reserve bandwith for it. The problem is as far as I can tell the N450 does not allow me to do this. The only QOS feature I saw applied to wifi. The VOIP device is plugged directly into the router. The VOIP device has it's own QOS feature. However in order to use it the device must sit in between the modem and the router. Since I currently have a combined modem/router it looks like this is not an option with my current setup. So I was thinking the best solution to my problem is change my setup so that I can do this. I was thinking of buying either a dedicated modem or dedicated router. Then using the N450 as the other role of either modem only or router only. Am I correct in assuming this is the best solution to my issue? And also is my assumption correct that the N450 can be used as a modem only or router only configuration? Do you have any advice on which role I should use it as: modem or router? Basically I was thinking one of these two routes:

n450 -> Ooma -> new wireless router

or:

new modem -> Ooma -> n450