Hi Jipper, welcome to the Seven Forums.
The external IP is for your line (router), so every device you have using the same line have their individual internal IP but one, common external IP. If you need different external IP's, you need to connect to Internet using different lines with each having their own router (or modem).
If a website, service or similar does only allow one login from one IP, they have reasons to do so. Quite often this is to assure you can not have multiple user accounts. This is why I am not willing to tell you any workarounds for this. Some other members might think differently, which I doubt.