Apps currently not listed in /e/'s Apps installer can be requested by you directly in /e/'s Apps installer. /e/ developers will then see to it that the requested Apps get listed if possible.
Not every App out there will work on /e/ because /e/ has no Google services. That’s the point of the exercise.
Aurora Store lists Texecom Connect as depending on Google services. If an App needs Google services to work, /e/ has microG in place as an Open Source replacement to try to cover that base as good as possible, but there are limits to this, and so not every App which needs Google services will work.
And even if such an App works on /e/ at a particular time, Google might break this with changes on their side any day, with microG having to catch up again, if at all possible. So there might be outages or an App stopping to work completely.