Reading from the reply to your request, I get a feeling back links are not a priority. I believe Agenda is fine as it is, regardless, with the manually linked notes.
In the meantime, the only two apps supporting back links that i can think of on iOS/iPadOS are Drafts and the mouthwatering and unbelievable Craft, the undisputed champ if you ask me, on Apple mobile.
Additionally, NotePlan 3 (digital bullet journal) has been out for two weeks and the developer, Eduard Metzger, said a few days ago he’ll bring feature parity with macOS in 1-2 weeks in terms of back links. He’s giving it full priority alongside tending to an onslaught of eMail with the release of the new app.