I'm no specialist
in Linux, but, I have had some of the same problems as you ealier on! Therefore here is 3 simple solutions. This is maybe not what you want to hear, but it is what it is
1. Move all websites from IIS to your Linux Webserver.. (I did it some years ago, it can be done) If you are hosting several sites which is not your own, then maybe you do not want to risc smashing the sites, I get that, then go directly to suggestion 2.
2. Get another IP number from your ISP, it can be done, I just did exactly that. For a few ekstra coins a month my ISP provided an extra static IP-number. If you are a private customer at your ISP they might be able to give you 1 extra static IP in addition to the IP you got togehter with your Router. By this I mean most private ISP's can provide 1 extra IP address - The one you got with your Router can be made static by choosing so in the Router setup - and therefore can not be regarded as an extra IP. Maybe you can use this argument to get another IP number which would solve your problem instantly.
3. Set your secondary server (less important or if it used for personal use) up to run on port ex. 81 instead of port 80 - then forward port 81 (portforwarding) in your router to the IP number for this server (ex. 192.168.0.101) I do not know if you are using NAT or how your setup is, but this is one way to do it. Then you can access the sites of this server by typing http://yourdomain.tld:81
I hope this will help you - usually I would not repply to anything in here due to my status as a novice, but in this case I had "some" knowhow to share..
I call upon some of the wizards in here to share some wisdom here