Generally, Beckhoff lists this in their docs, which you can get to via
beckhoff.com/EL5101
-> documentation. For the EL5101, this covered on
download.beckhoff.com/download/Document/...l5101en.pdf#page=273
. You *should* be able to factory reset it via `ethercat download -p XX 0x1011 1 1684107116`, where XX is your EL5101's slave ID.
Having said that, using TwinCat on Windows to reset things tends to work better than using `ethercat` on the command line.