Im not familiar with yr router/modem but have had similar issues before with other routers.
The problem will be in the router that much I know....often fixed with a Trail and Terror approach.
Some routers never work.
We have had occasions with 2 routers on different jobs set up the same way and one worked and the other wouldnt. wierd.
Try pointing JACEs gateway address to the router.(dont think this will make much difference)
Some routers seem to not like forwarding to IPs that aren't within their DHCP range so fix the JACE IPs and tell the router that the 2 IPs are fixed ... may even need to know the MAC address of the JACEs as well (no not the Bacnet MAC)
Sometimes routers want a re-boot after changes even though they don't say so ... typically DMZ changes and fixing IPs need a restart if the IPs that get fixed were formerly given by DHCP....reboot everything (router/jaces) and see if that helps
need to ask ... yr definitely typing
http://[myIPaddress]:80 and
http://[myOtherIPaddress]:81 .... and you have forwarded 81 too, right?
and you have the jace web services set up as ports 80 and 81 respectively too?
and the second JACEs fox service is definitely running on 1912?
...when yr trying from outside, have you dropped yr firewall or made pinholes for 81, 1911, and 1912 ?
...my networking voodoo could be stronger....thats all I got ...