Hy,
I have an internet box providing the 192.168.1.1/0 network (Orange)
Also setup the Mikrotik network 192.168.110.1/0 ( Mikrotik is having internet in 192.168.1.56 fixed IP), there is internet, all working fine

there is a VPN connexion on Orange Box, which takes the IP of 192.168.1.252
But there is no ping and no connexion in both ways ;
i need Mikrotik to let the computers in the box network (including the VPN) to see the other computers
i tried a few NAT rules but it didn't worked
thank you in advance

Its easy for computers behind the MT to reach other computers because all traffic out the MT is natted to the WANP of the MT .156, which is on the LAN of box devices.
Their return traffic goes back to the MT, and the MT un-sourcenats that back to the originators.

However consider the reverse, when the originators are on the same LAN as the MT. How do they reach an unknown subnet that the brandname box router is not aware of!!
Thus what you need to do is find out if you can make static routes on the BOX.
Like
add address=192.168.110.0/24 (or address of specific server) gateway=192.168.1.56

The problem is not on the mikrotik side!!

Now you could make port forwarding rules on the BOX router, but static routes are better.
would look something like --> add dst-port=56787 protocol=tcp address=192.168.1.56

On the MT ensure you have a firewall rule allowing port forwarding
add chain=forward action=accept connection-nat-state=dstnat

AND the port forwarding rules in NAT
add chain=dstnat action=dst-nat dst-address=192.168.1.56 dst-port=56787 protocol=tcp to-address=192.168.110.Y