ffs once and for all..
There are essentially four common types of ATC surveillance. Primary radar requires a return off the skin of the aeroplane = heaps of power out for a tiny reflected return = very limited range (< 100 nm). All other forms of surveillance (secondary radar/ADS-Broadcast/ADS-Contract) require a transponder in the aeroplane in order to get s return. SSR and ADS-B are ground based, so they are limited by line-of-sight rules (i.e range ~ 300nm), ADS-C is via satellite return and has essentially world wide coverage. No return on anything except Primary simply means the transponder is u/s