If your app isn't going to blast the server with thousands of requests every hour, then you could just do the request via normal http, at least to get your app back up and running while you find a more-permanent solution.
At worst, if you overrun your welcome at the server, your app will be blocked after too many requests from an IP address.
How many lookups are we talking about? Eg 100 users x 100 lookups per day = 10,000 lookups per day. Short term to get you going, that should be ok. But if your multiplication turned out to be 10,000,000 lookups per day, ok, that's probably going to get blocked, same as the API library you tried out.
If your app caches the lookups long-term then the number of lookups should decrease over time. In my use-case, doing 70 deliveries per day, it petered out after a couple of months to ~5 new addresses each day.
Not that it mattered for us: we have this beautiful dataset available:
and there were only about 100,000 addresses in our area, which was like a 10 MB text file that could be kept in memory no problem (same size as three camera photos). Maybe something similar is available for the area you're app is used in.