VPN error, size too small?

Question

VPN

There is a head unit in the Zyxel Giga office (KN-1010), a warehouse where Asus RT-AC1200 (padavan)
A VPN server -pptp is configured on Zyxel, asus is in the warehouse with a usb modem, configured a vpn client on it. Everything connects and the tunnel works. But after a while, the connection is lost for a couple of minutes, then restored. In asus logs:
kernel: mppe_compress[5]: osize too small! (have: 1358 need: 1508)
kernel: ppp5: ppp: compressor dropped pkt
pptp[2035]: No response to 6 echo-requests
pptp[2035]: Serial link appears to be disconnected.
pptp[2035]: Connect time 20.3 minutes.

Through telnet, I ran the command sudo ip link set dev ppp5 mtu 1500 - it does not help.