Hi,
Most of the work I do on Mikrotiks is related with VPN connections over mobile (2g/3g) networks. The situation as I observed it is worrying me. Let’s say we have a main router connected to the internet over FTTH 100/100mbit link. On the other side we have numerous routers (mostly 411, 411U and 411UAHR) connected through mobile 2g or 3g networks to the internet, using different modem brands and manufacturers (Sierra wireless, Novatel, ZTE, Huawei,…). Also those remote routers establish some type of VPN to the main router (SSTP, PPTP, L2TP).
Mostly all remote routers connect to VPN and I can see themn as connected on the main router. The problem shows itself when I start to produce network traffic over VPN. Very little data needs to be sent - around 15kb every 15 minutes. The VPN tunels disconnect and a lot of times they reconnect back but sometimes they dont and in logs on main router I see it came to authenticated, but when it should say what IP the remote router should have it disconnects… and so on. I have tested MTU from 1420 - 1500 and I saw no difference. The VPN links just aren’t stable (or I don’t know how to make them more stable). 
If I use APN from mobile provider I can see some latency, but never a disconnect. The 2g/3g signal on all locations is between -65dBi and -90 dBi.
One of those remote locations is my workshop where I also have a test 3g link (and PPTP VPN) to main router. It experiences the same simptoms as all other remote routers. But then I use my android phone and connect it in my computer via cable or wireless, using the same SIM card (it doesnt matter which) and I establish a Windows PPTP connection to Mikrotik main router it works as it should. I get no disconnects, I can even send and receive larger files (100MB+), again without disconnects.
WTH? 
Any help apreciated.
br,
bysaRD