Where are the cracks, not many fibreglass trays about would be my guess, but if all the "plastics" are a good colour match that would suggest all the same, the other give away is a fibreglass tray would usually be more rigid.
Hokey kokey. Gonna have a stab at this myself. I've looked on Youtube and seen several ways to weld with soldering iron. All my own previous bodges have been none weight bearing, but the shower tray will be
In the past I've melted along the crack and dragged it across in a criss cross type weave. Other ways are shown on Youtube. Some just melt along the crack and then fold in. Some create a channel each side and fold over.