Is there an error message or something?
Aurora Store (alternative access to free-of-charge Google Play Store Apps) lists “Nextplus Free SMS Text + Calls” as GSF-dependent, which means it needs Google services.
/e/ has no Google services. /e/ uses microG to mimic Google services as good as it can, but that has its limits.
While it works for a lot of Apps, there is no guarantee, and even if it works at a particular time, changes on Google’s side can break that at any given time, until microG did the necessary catching up again.
Nextplus is currently not listed as not working here … Editable List of Apps that work or do not work with microG … or here … https://github.com/microg/GmsCore/wiki/Problem-Apps … so your problem with it now might still be something else.