CG3000D v2 Wi-Fi slowdown/firmware not updating

Hello,

I have had the N450 router for over 4 years. I have had ups and downs with my Internet connection, but the issues have generally been on my ISP's end. Recently, I have been experiencing wireless Internet issues, so I contacted my ISP to check on my Internet connection. The ISP came and laid down a new line, but my wireless connection has not improved. The firmware on my router is V2.05.18, while the latest firmware listed for my ISP (Cox) for this router on Netgear's website is V3.01.16. I have restarted my router many times, but the firmware never seems to update. I know that the firmware is updated through the ISP, but the discrepancy between the router's firmware version and the ISP's latest firmware lead me to believe that my router stopped receiving updates a long time ago, if it ever did to begin with. 

1) Is there a way of checking whether or not my router is capable of installing firmware updates? (i.e. can I check if my router's firmware updating powers are broken?)

2) Are there any other steps I can take to fix my wireless speeds being so slow? I pay for 150mbps from my ISP, but I've been getting around 10-15mbps wirelessly when I run a speed test. This is pretty bad. I've also already went through all of the steps on the general troubleshooting page: https://kb.netgear.com/27322/Troubleshooting-internet-connectivity-issues-for-NETGEAR-cable-modem-router.

Any help on this would be greatly appreciated. Another visit from the Cox support team might result in a $75 charge, and I'd like to avoid that mess entirely if my router's busted and I need to buy a new one.