Plastic tanks are made of PolyEthylene(PE), a first cousin of Teflon(PtfE). They belong to the NonStick family.
PE can be melted but not glued without extreme prep measures and exotic solvents, and with mixed results.
This is a non-starter. Don't bother, unless you enjoy failure.
If it is a Moeller Tank, Give them a call.
If if broke under normal use conditions, they will commonly send a free replacement despite being years beyond the warranty period.
They really back their products! The last thing they want to see is a fatal accident with their name even mentioned in the report.