IM151-8 PN plc's website does not support MTU size less than 1500byte

Accessing IM151-8 's website through VPN is impossible.

Without VPN (through LAN) all normal.

When using ADSLrouter---PPPoE---VPN---GPRSrouter, the answer from website consists of 1,43KB instead of 16KB (see in attachment).

So I draw a conclusion that it is exactly size of MTU (Maximum transmission unit), which configured on routers by default 1492 bytes. And PLC's website doesn't support neither "Path MTU discovery" nor "Permitting Packet fragmentation". All this making impossible to use website, it doesn't load...

Help me plz to solve this trouble, how can I load webpage?

PLC order number ...-8FB00-... firmware 2.7.1.

Maybe new8FB01 resolves this problem, but I'm interested in solving on old PLC -8FB00.

Temporary solution - is to reduse MTU sizeon SCADA computer.

It solves problem.