Which FOSS FTP client app for android?

Hi,

till ca. 2018 I used clouds like Dropbox or Google Drive for storage. But since then I’ve moved more and more to FOSS, privacy-respecting software. Now I have a router with an USB port and built-in FTP (doesn’t support SFTP) server. My plan is to backup some apps, pictures, settings etc. I would also store a copy of the most important files at a nextcloud instance. At the moment I don’t have enough money to spend, otherwise I would have bought a NAS.

Q: Which FOSS android apps are available to up- and download from an FTP server?

Maybe you’ve also a little guide for me, as I was using only cloud storage for years and therefore have almost zero experience with this stuff.

Thank you :)