Look into the 3-2-1 strategy. Also: At least one Backup should be taken offline after the backup is done. This might be done via Tapes on a Tapelibary, where you would put your Used tapes into a fireproof safe (certified for Tape fire protection - ask me if you dont know what that means). Those backups that are not connected to a network are most reliable in such a scenario. Most encrypters encrypt right away and thus offline/archived backups are most likely not already affected.
If your trojan was keeping itself silent for a couple of months (some specialised do that) even your archives are at risk. In such a situation mostly the only solution is to build from fresh.