yes that sould work, we have theses phones in a few remote locations, however would not recommend them for business usage, they seem to break down within 6-9 months of usage.
no need to enter the mac address as the phones are not directly support by 3CX, however providing the both routers are setup correctly, you should have no issues.
can you install the 3CX softphone on a remote pc and see if you can make and receive calls ok?