Router is used in 3G/4G mode.

My internet provider is using MTU size 1392; The default size of WLAN interface of the router is 1500 and this makes me troubles with VPN and remote desktop of Windows.

How to  change the MTU setting?

I've already tried suggestion from https://community.tp-link.com/en/home/forum/topic/85446 (set MTU for standard router and go back to #G/4G mode), but it didn't work - this firmware uses separate MTU settings for all connection times, I've changed all of them with no solution of my problem.

Now I am putting SIM card to my old Android mobile and using this device as a router - works perfectly with VPN and remote desktop. But this is not the final solution, I want to use MR6400.

Please help me!