Cannot connect to Heroku MongoDB - mongodb

Question

I created an Ionic app with NodeJS backend and Mongo database and deployed it to Heroku (with mLab MongoDB add-on). It worked well for a day. The next day I was unable to connect to the DB. The app went to sleep in the meantime, but when I opened it in the browser it woke up. I know that because I saw the static text that I put on the index page. But a few seconds later, an error page appeared. From the console and network tabs output I could figure out that the app is unable to access the database and to fetch data.
I found a post on the internet where the author said that he had the same issue and that he deleted the db instance on Heroku and created a new one. So I did the same and then I was able to start my app normally again.
Today the same thing happened - I can't connect. When I switch the DB URI in the code to mongodb://localhost/mydbname and run the application locally, it works perfectly, so I concluded that the app code itself is fine, there's only an issue with connecting to the DB. It also wouldn't work locally if I try to target the DB on Heroku.
I thought that I might forgot the credentials, so I went to the Heroku dashboard and changed both the username and the password. Then I tried to connect through the console with mongo ds<number>.mlab.com:<port>/heroku_<name> -u <username> -p <password>. Here's the output:
MongoDB shell version v3.6.3
connecting to: mongodb://ds213239.mlab.com:13239/heroku_d3xjtj3x
2018-03-16T16:09:18.215+0100 W NETWORK [thread1] Failed to connect to 54.236.230.76:13239 after 5000ms milliseconds, giving up.
2018-03-16T16:09:18.216+0100 E QUERY [thread1] Error: couldn't connect to server ds213239.mlab.com:13239, connection attempt failed :
connect#src/mongo/shell/mongo.js:251:13
#(connect):1:6
exception: connect failed
Currently I'm using free version of both hosting and DB add-on, because I want only to test the app and to show it to my client (who will then decide if he wants to pay for the Heroku services). If I have nothing to show and if this happens each time, well, that would suck.
And here's the fun part: I wanted to try to open the app one more time so that I could copy and paste the exact error message here. And this time it worked normally! (???) The only thing I did is that I changed the username and pass on the DB dashboard, but I didn't push the updated code and I also didn't change it in the MONGODB_URI config variable.
Does anyone have an idea why does this happen? Is there a way to keep the DB awake and accessible (without paying extra)?
Thanks.
EDIT: It doesn't work again!!! Literally 30 seconds after submitting this post I tried to access the app again and it didn't work! There is an error page which says:
Error: Uncaught (in promise): [object Object]
at c (http://localhost:8100/build/polyfills.js:3:19752)
at c (http://localhost:8100/build/polyfills.js:3:19461)
at http://localhost:8100/build/polyfills.js:3:20233
at t.invokeTask (http://localhost:8100/build/polyfills.js:3:15660)
at Object.onInvokeTask (http://localhost:8100/build/vendor.js:4973:33)
at t.invokeTask (http://localhost:8100/build/polyfills.js:3:15581)
at r.runTask (http://localhost:8100/build/polyfills.js:3:10834)
at o (http://localhost:8100/build/polyfills.js:3:7894)
at e.invokeTask [as invoke] (http://localhost:8100/build/polyfills.js:3:16823)
at p (http://localhost:8100/build/polyfills.js:2:27648)
ONE MORE EDIT: I deleted the DB again, created a new one, changed the connection string in my app and pushed the changes. Now it works normally. I'll check again tomorrow and update this post.
NEW EDIT: One day later, the app doesn't connect to the DB again. And again I get the same error message.
Wtf, Heroku???