PS3 says "The router in use may not support IP fragments...?

I bought my PS3 last year on Black Friday, and I had one for 2 years before then. I have played online with them the entire time using the same setup. It is a relatively crappy setup, but it has always worked until now.

Broadband Internet

>>

Belkin Router 802.11 n

>>

NetGear WiFi Extender

>>

Direct Ethernet connection to extender

The error message appears when I try to test connection. It says "The router in use may not support IP fragments, and the communication features of some games may be restricted. For details, contact the router manufacturer." The real in-game issues I face is when I try to play online, I get very severe intermittent lag. On Black Ops II, I will jump from 1 red bar to 4 green bars regularly, often skipping the second and third bar.

I have already tried tweaking my MTU down to 576 instead of 1500, and the issues still appear. (I tried this hoping smaller packets would prevent fragmenting - I suppose it was just wishful thinking)

The connection screen reads:

Obtain IP address: Succeeded

Internet Connection: Succeeded

PlaystationNetwork: Succeeded

UPnP: Enabled (or Disabled, I've tried both)

NAT Type: Type 2

Connection Speed (DL): -

Connection Speed (UL): -

Then of course the error message scrolling at the bottom of the page.

I tried contacting both Belkin and NetGear, neither has any idea what the problem is nor can they offer any advice. Sony doesn't have a clue, and they just tell me to call Belkin and NetGear even when I told them they can't help.

Any help would be appreciated.