s5200 ipmi serial over lan / sol

s5232-on and s5248-on with nvidia cumulus linux, ipmi bmc gets ip over dhcp, I can read sensors, power cycle, add ipmi users ... all as described in dell manual :

S5200-ON Series BMC User Guide March 2019

only missing funcitonality is Serial Over Lan.   If I activate it drops out, and later deactivate doesn't release the previous broken session.  If I search on GNU/Linux servers there is some GRUB config to redirect console to ipmi/bmc, so I *guess* some ONIE config is needed to do the same on a Linux Switch?

Nvidia and Dell support is as clueless as myself