The premium APIs are just for automatic categoring at the source (i.e. you download your transactions with categories already). Firefly III does support automatic import using Nordigen by setting up a cron job which automatically POST the transactions downloaded using Nordigen API. You will have to setup the Nordigen connection only the first time you do it, then download the json and POST the correct url. There's a section in documentation of the data importer which explains exactly that.
The premium APIs are just for automatic categoring at the source (i.e. you download your transactions with categories already). Firefly III does support automatic import using Nordigen by setting up a cron job which automatically POST the transactions downloaded using Nordigen API. You will have to setup the Nordigen connection only the first time you do it, then download the json and POST the correct url. There's a section in documentation of the data importer which explains exactly that.