I have an internal Nextcloud Server that I can access through VPN/Wireguard. The Nextcloud android client can edit files (either in plaintext, which I prefer, or markdown rendered mode). For when I have no mobile internet access, I use Markor [1], installed from F-Droid. The files in Markor are synched to Nextloud, when online or in home wifi again.
[1]: https://f-droid.org/de/packages/net.gsantner.markor/