I got it working through a dstnat Rule, but with this I was not able to pass the external IP to the Router,
but I had to use a seperate internal IP for the 3G Modem Router.
Here is the Setup I am using:
add apn=internet default-route-distance=1 disabled=no name=ppp-out1 port=usb1
add bridge=br-drei name=profile-drei
/interface bridge port
add bridge=br-drei interface=ether2
add bridge=br-drei interface=ether1
add name=dhcp_pool1 ranges=192.168.124.2-192.168.124.254
add address=192.168.124.1/24 interface=br-drei network=192.168.124.0
/ip dhcp-server network
add address=192.168.124.0/24 gateway=192.168.124.1
/ip firewall nat
add action=masquerade chain=srcnat
add action=dst-nat chain=dstnat dst-address=188.8.131.52 to-addresses=\
add disabled=yes interval=1d name=Reboot-daily on-event="/system reboot " \
And on the Router where the 3G Modem Router is connected, you just put a static IP in the same IP Range (192.168.124.2), with it's masquerading rule.
I have a static IP Address for my 3G Modem, not sure how this should work with dynamic Addresses.
Hope it helps.