I did a backup and restore of my Signal stuff two times a short while ago … once when I switched from Signal to Langis, and once again when I installed 1.5-r from scratch because of microG trouble with 1.5.1-s.
Did a Signal/Langis backup, kept the long number given by Signal/Langis at hand, did the restore with entering the long number, everything was back, no trouble to report.