I switched from provider and I am stuck with a stupid error that I can't find.

The provider has given me a /27 range of address.
They added one of their Router to act as the gateway taking one of my address for them. They use the .65
I have a Mikrotik after with the first address and this works fine which ends in .66
What I am trying to do is use the second address .67 but forward it to another mikrotik which is connected to another port on the first mikrotik.

Before, I was only adding Routes but unless I add the .67 in the list of Address in the first table, their router doesn't seem to know that we exist.

I tried adding it in the address so it responds to the ping. Ok for the .67 but I don't want it to answer to ping on my first Mikrotik.

I Added a NAt rule to DSTNAT .67 to DSTNAT the second router and did the opposite for SRCNAT. Nothing

Any idea?

Just add both MT routers to the same switch and no other mangling is needed. You can make a switch by bridging 2 MT ports together and assigning proper addresses to all connected devices.