How to get DRBD nodes out of Connection State StandAlone (and WFConnection)?

The situation was apparently caused by a case of split-brain.

I had not noticed this because I had only inspected recent journal entries for drbd.service (sudo journalctl -u drbd), but the problem apparently was reported in other kernel logs and slightly earlier (sudo journalctl | grep Split-Brain).

With that, manually solving the split-brain (as described here or here) also resolved the troublesome situation as follows.

On split-brain victim (assuming the DRBD resource is r1):

drbdadm disconnect r1
drbdadm secondary r1
drbdadm connect --discard-my-data r1

On split-brain survivor:

drbdadm primary r1
drbdadm connect r1