Hi, which firmware did you upgrade to? You can have a try to downgrade the firmware or do some troubleshooting with TP-Link support.
Perhaps you issue will be solved.
I found a link for how to recovery the firmware:
http://forum.tp-link.com/showthread.php?81462-How-to-recovery-the-router-when-you-bricked-it&highlight=fitz

hi Kevin, I've edited my first post, and mentioned firmware version.

I don't know, maybe upgrading firmware has changed something in the routeur's settings, something I couldn't fix myself because I'm no advanced user.

Maybe someone could lend me a hand?

You can contact support.fr@tp-link.com for help.

Anton L.Z. wrote

You can contact support.fr@tp-link.com for help.

I did contact them, as a reply they sent me some crappy PDF tutorial on how setting up a wireless and wired network. How useless. Tp link have bad quality products, I won't buy from them in the future.

Actually if your product is still under warranty, you can ask for a replacement.

I'm having the same issue with my C8 V1 running the older 2015 firmware.

I just upgraded to 1gig fiber and at the ONT I get 970+ on download and upload. But putting the C8 between them limits the download to 280 and upload to 750-800

Upload looks good, but download seems slow. Downloading large files ranged from 2MB/s to 30MB/s. Very inconsistent speeds.

NAT Boost is on

Anything else I should check?

suggest you to turn off NAT Boost and upgrade the firmware.

I have a C8 V1 and also and upgraded to the new firmware. I went to the new version of firmware hoping I would be able to sign on to the modem through the C8 as it was one of the many things support had me do. After the firmware upgrade, Port forwarding stopped working for my security DVR. Started a new support ticket and never was able to get port forwarding to work. I should note that the new firmware changed the GUI interface so things are completely different and you are required to reenter all your parameters. So I downgraded to the old version and Port forwarding works fine now. Still can not sign on to the modem unless I bypass the C8. So you can downgrade if you need to, it is just like upgrading but using the older file.