Do you know what are the parameters ELYup, ELYlow, FCup, and FClow which are the SOC of the electrolyzer at which the fuel cell and the electrolyzer switch ON and OFF in HOMER?

HOMER uses no such set points.  The electrolyzer operates whenever the surplus electricity exceeds its minimum load point and there is headroom in the hydrogen storage tank.  The fuel cell operation is governed by need, fuel supply, schedule, and its cost relative to the other dispatchable power sources.