Hello i have recently purchased the draytek vigor 2850n.
I'm with bt infinity for business and have the latest firmware installed on my router.
Everything about this router is very good apart from the mtu which for some reason keeps reverting back to 1442 from 1492.
I have set mtu to 1492 using the telnet and browser but it still eventually reverts back normally within a few hours.

I would really appreciate any help or advice.

Many thanks in advance.

2 ways to change it

1- Wan>internet access>details page have you tried to set it there via the UI?
Also what command are you trying to use?
2- The correct one is "wan mtu" wan 1 and wan mtu2" for wan 2

Latest firmware is 3.6.4.1 but you may need to use the *rst* file if you load the firmware and do all settings manually back from the begin avoiding firmware's missmacthing
ftp://ftp.draytek.com/Vigor2850/Firmware/v3.6.4.1/