None of the following is needed. Just have your remote phones register to 9500…
None of this is needed:
Change remote site 1 extensions to register as 9500, 9501, and 9502 in FreePBX extension settings
On main site router, port forward 9500, 9501, and 9502 from internet to FreePBX server
At remote site 1:
Change phone configurations so that
Phone 1/ext 1 registers to 9500
Phone 2/ext 2 registers to 9501
Phone 3/ext 3 registers to 9502