The problem you are encountering is that WordPress and your browser are interpreting everything after the “#” as a client side hash. In other words, PHP and WordPress never see the “?tier=Test1” bit.
There are few ways to fix the issue though I’m not sure which will be most efficient in your situation:
1) Trigger your modal with a JS click event instead of relying on the href attribute.
2) Store your parameters in a different attribute such as ‘data-rel’ and use JS to pull the parameters after the modal loads.