These things are archaic. I believe the signal meanings have not been updated (translated to modern terms) since WW2.
Effectively a flashing green means "continue approach." (It says "return for landing" A flashing white given to an aircraft in the air is less equivocal; "return to land at this aerodrome").
A steady red means, depending on your best guess if it happens, "orbit", or "circle the aerodrome".
Doesn't everyone carry cellphones these days? Last genuine radio failure I experienced as a controller we simply rang the aero club, asked for the students mobile number, gave her a call, and subsequently an approach/landing clearance.
I'd learn the meanings verbatim for the theory test, but in the real world, rely on something else, frankly.