PPTP server drops client every 10 minutes

I have mikrotik RB750Gr2 with 6.43.4 RouterOS.

Where is PPTP Server with assigns addresses from subnet 192.168.4.0/24. Mikrotik net is 192.168.21.0/24

I connect CentOS client to mikrotik using this manual http://valec.ru/?p=174

Everything works ecxept client disconnects every 10 minutes. It’s like the router doesn’t send keep-alive packets. Or somehow centos server doesn’t receive keepalive packates.

What is wrong and what can I do to make the connection persist?