https://matrix.to/#/#koreader_koreader:gitter.im from matrix prospective this is the address.
Search #koreader_koreader:gitter.im
Or simply click on gitter badge hyperlink from KOReader's GH :
https://github.com/koreader/koreader...-e-ink-readers It should open in your preferred client. And yes you can join from whatever server that your account is signed in, that's the whole point of decentralized platform.