How to allow for connecting to external VPN from LAN?

RouterOS Beginner Basics
1

Hi,
The problem is as following:

  1. From location ‘A’ I can connect to established company L2TP VPN in location ‘B’ using internet provider cable modem and built in router - provider settings;
  2. From the same location ‘A’ I cannot connect to the same L2TP VPN in ‘B’ after I connect, using the same computer connected to RB1100 Eth2, so via LAN. RB1100 is connected to cable modem & router via WAN interface defined for Eth1 of RB1100.
    Are there any limitations for VPN pass-through packets I should address in Firewall configurations?
    Of course WAN access for http is available and works perfectly. However I need to connect sometimes via VPN to someone in the ‘external’ world.
    I am a beginner - thank you very much in advance for advise!
    Best,
    Maciej