I'm really sorry to have been misunderstaken. When I only used Amiga i was quite expert on miami, after 10 years I forgot almost everything, I wanted to try the solution you suggested to me, I really appreciated it.
Then I didn't configure a manual route, i put the static IP address 192.168.1.200 and pinged it, this is the result:
ping wrote 192.168.1.1 64 chars, ret=-1
thats a bummer. Ideally what you wan to see is a response in milliseconds.
well, you have manually put in your subnet mask 255.255.255.0 and your router in as the gateway then I don't think you should have to do anything else as MIAMI will add the gateway as the necessary default route.
I'm not sure if your issue is that you are behind a double NAT and you need to specify a different subnet for your IP (ideally you really want your airport to act as a bridge, but save that for a different day) if I understand your setup correctly.
Meaning your primary router is DHCP server and provides NAT
192.168.100.1
then your secondary router is 192.168.100.100 or something and it also does NAT
computers under that would then be 192.168.

.

plug in your macbook and determine its DHCP address... then remove it completely and then put the Amiga on the exact same address. If that works then you'll pick a slightly different address(not in DHCP range) for your Amiga... If that does not work you might be out of luck. I don't see why it won't work though. Apple airports are quality hardware...