I checked the XOOPS installation guide, but it assumes that everything will be installed correctly.

I don't know how common my setup is -- i have a registered domain, miabrown.com, that is forwarded to my server (a pc connected to the internet through a wireless router) from a web hosting service. Luckily, my IP address has only changed twice in the past year. Anyway, when I installed XOOPS, it used my internal IP address (192.168.###.###) for XOOPS_URL. In order to view my site from outside the network, I have to change the IP address in mainfile.php to that of the router.

As a workaround, I have two mainfile.php.### files that I alternately copy to mainfile.php to switch my site back and forth (between internal and external viewing). If I use the domain name in the mainfile.php, all I see is an un-themed site, internally and externally. There are issues with the banners, too. I have to manually change all the banners to the internal or external IP address. Is there a better way to set up my system so that I can see a properly themed site no matter where I'm viewing it from? Perhaps Apache is not configured correctly or /etc/hosts? Since I haven't seen other posts with this problem, I'm assuming that I haven't set up my system correctly, though I don't have this problem with other content management software.

Any assistance is appreciated.

If only real-IP changed, not hostname and you can assign (over internal DNS-zone or hosts file) any name to 192.168.* address

you can use trick "Answer on any host request, which reached XOOPS-site, with requested name"

Just edit one string in mainfile.php and write it as
define('XOOPS_URL', 'http://'.$HTTP_HOST);

(this way I have one site known under 2 different names to Net, and all internal links are changed according to requested hotsname)