Before you buy the full version of "L2TP connect", please test with this free demo app.
"L2TP connect" is a VPN Client for raw L2TP Servers without IP-Sec. It uses Android builtin VPN Service so you do not need to ROOT your phone to gain this feature. You can buy "L2TP connect" in this market.
To Samsung 6.x users:
On Samsung 6.x VPN is reported not to work unless the VPN app is exempted from Powersave features. You should disable it by yourself.
Settings -> Battery -> App power saving -> Detail -> L2TP Connect -> select "Turned Off"
Settings -> Battery -> Power Saving Mode -> "Restrict all background data": Off