OpenSearch was removed in the 2019 Firefox for Android remake and still not brought back. Just look at the steps one needs to take to add a search engine now — this is not normal. It also makes engines other than the default ones second-class citizens, without even their own icons.
If your browser is configured to default to private browsing, this will require you to log back in to Kagi each time your tabs are closed. To get around this, in the Search string to use field, use your session link and append &q=%s at the end.