PPTP-client doesn't work properly

Hi. I use RB951G-2hnd for home with the latest firmware.
So, I connect the internet via PPTP. I used TPLink router before, but I decided to change it to Mikrotik . I had an issue with native firmware on my TPlink and I couldn’t access some sites (speedtest.net …) but later time it has magically gone. I had been using it about a year and then decide to change it to Mikrotik (because TPlink slows down the Internet speed). I’m new with this RouterOS and routing so I read some basic manuals and configure Mikotik like in this http://strongvpn.com/setup_mikrotik_pptp.shtml article.Unfortunately I don’t have network settings. Provider grants only .exe file that creates PPTP connection on windows. The only info I could get from it are server Hostname, user and password and MSchap2. So I configured mikrotik and lucky for me speed grows up to 90MiB as it’s supposed to be without the router (from Windows 7). Seems like everything works fine but I can’t access some sites ( I don’t receive the packages from them at all: speedtest.com howtogeek.com nekto.me.. ) and some others are loaded fully instantly but browser shows they’re being downloaded for a few minutes.
Here’s my /interface pptp-client> print

Flags: X - disabled, R - running
0 R name=“nexus_vpn” max-mtu=1500 max-mru=1500 mrru=disabled connect-to=172.25.1.251 user=“nightmare”
password=“nightmare” profile=default-encryption keepalive-timeout=disabled add-default-route=yes
dial-on-demand=yes allow=mschap2

I think it’s about MTU or MRU but I already tried to lower the MTU and it didn’t help. I also turned on pptp logging but there’s nothing useful there. Anybody has any clues?

And one more question. How to tell router that VPN is my main internet connection. My ntp client doesn’t work and I can only ping sites from router with “interface=nexus_vpn” option. I thought that’s “add default route” option on vpn… but probably it’s not.

Solved via iptables iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu