Do you have additional information on the 5 vulns it found? I've only found 2 DOS issues that that device has in regards to the web interface. The DOS may only be that which would be a nuisance on a home network but obviously more of an issue on a business network. Keep in mind that a DOS may actually be more than that, it could be that the researcher just didn't do enough work to get code execution and stopped at the DOS.

If there are no firmware updates for that device then you wont be able to remediate those vulns. However, you can mitigate your risk by making sure that the web interface is not exposed externally or using a different OS on the device like you mentioned. DD-WRT is actively maintained but like anything else, has its own issues:

http://www.exploit-db.com/search/?action=search&filter_page=1&filter_description=dd-wrt&filter_exploit_text=&filter_author=&filter_platform=0&filter_type=0&filter_lang_id=0&filter_port=&filter_osvdb=&filter_cve=

If I were you... I'd try to bust into that thing and see for yourself how big of a risk those vulns are!

If this is on your home network, on the LAN side then don't worry about those. If you're not using telnet to your router then turn that service off. Rule of thumb....turn everything off and then only turn on stuff you need.

If this was from the outside in, you'd have some issues but these are minuscule compared to other issues you may encounter on your network, IE bad wifi security, poor client security/patching etc etc etc.....

It certainly is. There may be a vulnerability, but if there is no measurable threat, then you have no risk so you're probably better off focusing on other issues.