Ubuntu VPN Dropping Out When connecting over mobile phone

Log Message

Sep 26 13:12:30 dime NetworkManager[21052]: ** Message: nm-pptp-ppp-plugin: (plugin_init): initializingSep 26 13:12:30 dime pppd[21749]: pppd 2.4.7 started by root, uid 0Sep 26 13:12:30 dime NetworkManager[21052]: Using interface ppp0Sep 26 13:12:30 dime pppd[21749]: Using interface ppp0Sep 26 13:12:30 dime NetworkManager[21052]: nm_device_get_device_type: assertion 'NM_IS_DEVICE (self)' failedSep 26 13:12:30 dime pppd[21749]: Connect: ppp0 <--> /dev/pts/2Sep 26 13:12:30 dime NetworkManager[21052]: Connect: ppp0 <--> /dev/pts/2.....Sep 26 13:12:34 dime NetworkManager[21052]: <info>  [1506427954.7518] manager: NetworkManager state is now CONNECTED_LOCALSep 26 13:12:34 dime NetworkManager[21052]: <info>  [1506427954.7520] manager: NetworkManager state is now CONNECTED_GLOBAL.....Sep 26 13:12:34 dime NetworkManager[21052]: <info>  [1506427954.7522] dns-mgr: Writing DNS information to /sbin/resolvconfSep 26 13:12:34 dime dnsmasq[21285]: using nameserver 212.23.6.100#53Sep 26 13:12:34 dime dnsmasq[21285]: using nameserver 212.23.3.100#53.....Sep 26 13:12:34 dime whoopsie[1451]: [13:12:34] The default IPv4 route is: /org/freedesktop/NetworkManager/ActiveConnection/7Sep 26 13:12:34 dime whoopsie[1451]: [13:12:34] Not a paid data plan: /org/freedesktop/NetworkManager/ActiveConnection/7Sep 26 13:12:34 dime whoopsie[1451]: [13:12:34] Found usable connection: /org/freedesktop/NetworkManager/ActiveConnection/7Sep 26 13:12:34 dime whoopsie[1451]: [13:12:34] onlineSep 26 13:12:34 dime dbus[807]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'.....Sep 26 13:13:16 dime pptp[21769]: nm-pptp-service-21742 log[logecho:pptp_ctrl.c:719]: Echo Request received.Sep 26 13:13:16 dime pptp[21769]: nm-pptp-service-21742 log[ctrlp_rep:pptp_ctrl.c:259]: Sent control packet type is 6 'Echo-Reply'.....Sep 26 13:13:45 dime pptp[21754]: nm-pptp-service-21742 warn[decaps_gre:pptp_gre.c:347]: short read (-1): Message too longSep 26 13:13:45 dime pptp[21769]: nm-pptp-service-21742 log[callmgr_main:pptp_callmgr.c:245]: Closing connection (unhandled)Sep 26 13:13:45 dime NetworkManager[21052]: Modem hangup


Tried

Downgrade network manager:

sudo apt-get install network-manager=1.2.2-0ubuntu0.16.04.4
sudo service network-manager restart

Set VPN to connect with Automatic VPN
Add 192.168.11.1 as the Additional DNS

None of the above seemed to have any effect.

Solution

In Network Manager in the systray, set the MTU to 1100 for the mobile phone connection.
Once connected, the MTU appears to be set to a minimal 1280:

# ifconfig wlp2s0
wlp2s0    Link encap:Ethernet  HWaddr 48:d2:24:a7:51:87 
          inet addr:192.168.43.180  Bcast:192.168.43.255  Mask:255.255.255.0
          inet6 addr: fe80::e3f6:2356:cf9d:d367/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1280  Metric:1
          RX packets:2546622 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1796946 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:2423974523 (2.4 GB)  TX bytes:1025131303 (1.0 GB)

# ifconfig ppp0
ppp0      Link encap:Point-to-Point Protocol 
          inet addr:192.168.11.108  P-t-P:192.168.11.1  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1400  Metric:1
          RX packets:12753 errors:0 dropped:0 overruns:0 frame:0
          TX packets:12572 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:6992532 (6.9 MB)  TX bytes:984659 (984.6 KB)