After installing the database, you can connect to it using a standard ngrok TCP endpoint sending traffic to port 5432. 
ngrok tcp 5432
TCP endpoints are only available on a free plan after adding a valid payment method to your account.
Once ngrok is forwarding traffic to your PostgreSQL service, you can use the TCP address provided to connect to it remotely. 
psql -h 0.tcp.ngrok.io -p 12345 -U postgres -d postgres