Why my WR21 disconnects after 2 minutes?

I setup a PPP client connection to a PPTP server running on a linux machine. The server looks to work fine, since I can connect with mobile client and Windows PPTP clients.
My problem is that WR21 establish a connections with PPTP server, gets ip address, and it look to be working fine.
But after 2.0 minutes it drops the connection.

WR21 log file reports:

15:10:54, 02 Aug 2019,PPTP Tunnel 1 down
15:10:54, 02 Aug 2019,PPTP Call 1 down
15:10:54, 02 Aug 2019,PPP 2 Out Of Service,Activation
15:10:54, 02 Aug 2019,PPP 2 down,LL disconnect
15:08:42, 02 Aug 2019,PPP 2 Available,Activation
15:08:41, 02 Aug 2019,PPP 2 up
15:08:38, 02 Aug 2019,PPP 2 Start IPCP
15:08:38, 02 Aug 2019,PPP 2 Start AUTHENTICATE
15:08:38, 02 Aug 2019,PPP 2 Start LCP
15:08:38, 02 Aug 2019,PPP 2 Start
15:08:38, 02 Aug 2019,PPTP Call 1 up
15:08:37, 02 Aug 2019,PPTP Tunnel 1 up

Server logfile:

Aug 2 pppd[12491]: local IP address x.y.z.k
Aug 2 pppd[12491]: remote IP address x.y.z.n
Aug 2 pptpd[12490]: CTRL: Session timed out, ending call
Aug 2 pptpd[12490]: CTRL: Reaping child PPP[12491]
Aug 2 pppd[12491]: Modem hangup
Aug 2 pppd[12491]: Connect time 2.0 minutes.
Aug 2 pppd[12491]: Sent 9828 bytes, received 9744 bytes.
Aug 2 pppd[12491]: MPPE disabled
Aug 2 pppd[12491]: sent [LCP TermReq id=0x3 “MPPE disabled”]
Aug 2 pppd[12491]: Connection terminated.
Aug 2 pppd[12491]: Exit.

I tried to change lots of parameters, but none fixed the issue.
Is there any place I have to look for?
Firmware is the latest but with also the previous one I had the same issue.

Thank you.