Openconnect packet fragmentation and assembly issue


I have an odd issue here with OpenConnect VPN on the OpenWRT.
VPN Client works fine, it connects and it do the job, but it gets mad after certain volume of the traffic, as fast as the MTU max reaches out, it starts to drop the packets.

OpenConnect server ----------Internet-----------OpenWRT(OC_Client)---Lan(laptop)

tests have been done so far:
1-Test from OCServer to the WAN ip of the OpenWRT was on even with big ICMP sizes. Fragmentation is successfully done and also the assembly.
2-Ping test from OCserver to the VPN client IP address is successful but up the the MTU size, above that it fails.
3-Same result for ping from OCserv to the LAN ip address of the OpwnWRT.

I can do all sort of test if you need more investigation.

Just to say that I am completely positive that this is the MTU (frag./Assemb.) issue, because for example ssh also to the VPN Client works file up the the time that SSH doesn't have big output, but as fast as you read a big file or make a big output, it hags.