OK, so the IP of my content server is in the form of
http://127.x.x.x:8080/, however I tried that (and adding a opds after the last /), none of them worked in Aldiko, they just opened up my default mobile browser which led to a dead link. However the IP works perfectly fine on my desktop browser (I can view the catalog in FF).
If anyone knows how to get the 1st method (using Dropbox + Calibre2opds) working with their Android phones, please let me know. I can generate a catalog.html file just fine, it's just that when I try to connect to it as a custom catalog on Aldiko it always FC.