I want to communicate between the LAN user of my Mikrotik and the TP-Link router.
The TP-Link is already a LAN user of the Mikrotik router, connected by a LAN interface from MikroTik to the WAN interface of the TP-Link router.
Without any extra configuration, the TP-Link LAN user is able to ping all of the MikroTik LAN users (192.168.100.0/24), but the MikroTik LAN user can't ping the TP-Link LAN user (10.0.0.0/24).
To access the TP-Link user from the MikroTik LAN, I do the following configuration on the MikroTik:
IP>Routes
add dst-address=10.0.0.0/24 gateway=192.168.100.100 (192.168.100.100 is the WAN IP of the TP-Link router).
IP>Firewall>Mangle
add action=accept chain=prerouting dst-address=10.0.0.0/24

After this configuration, I can ping the gateway of the TP-Link LAN, which is 10.0.0.1, but I can't ping the users on the LAN side.

What I'm doing wrong here or how can I do what I want to do? Can anybody help me out?

Sure the MT Router doesnt know about the TP LINK subnets.
Not sure why you want doublenat and what purpose the TPLINK can do that the MT cannot ???
Why are you mangling traffic ???

Your route seems okay.

Would have to see a network diagram and full /export of the config.