Please make some picture on your network setup so it's easier figure how your network should working.
Okay i try figure how this shoud working, example:
Site A - B connected with Ethernet cable, A (ether1) to B (ether2 what is also bridge1). Site B connected via ether1 to DSL modem ethernet port?
ether2 / bridge
from DHCP 192.168.1.2/24 (example)
So A - B working because it's same /24 network but because DSL modem is different network you should make src-nat to Site B. Then you can ping. Then DSL modem can return package back to A. It think this is solution?
Add this to Site B:
/ip firewall nat add action=masquerade chain=srcnat out-interface=ether1