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Ever since I got my C5400 last fall I kept battling with internet connection speeds.

It doesn't matter if I connect via 5Ghz Wifi, 2.4Ghz Wifi or via LAN, I never get connection speeds faster than around 90Mbit/s and it fluctuates massively.

My ISP is Vodafone here in Germany and as I was convinced that the issue couldn't possible lie with such a high end router, I had them exchange the modem twice.

Turns out I get the full bandwidth of 400Mbit/s down and 25Mbit/s up when connecting my PC directly via LAN to the modem; alas via the C5400 it continues to fluctuate.

Is this a known issue and if so, are there recommended steps I should take? I'm prepared to tinker, but if it turns out that I cannot solve the issue myself, I will have to return it as I'm still within my warranty.

Thanks for any help provided!