For a plenary indulgence, doesn’t the Rosary have to be recited in a group (of 2 or more) or in a church building? I seem to remember hearing that a while back.
There are multiple ways to theoretically obtain a plenary indulgence,
And there are multiple ways to theoretically obtain a plenary indulgence where the Rosary is somehow involved.
Praying the Rosary privately (but not stopping between decades).
Praying the Rosary publicly in a group.
Join the Confraternity of the Rosary and there are even yet more ways.
Very possibly ways that I have not considered.
For example, perhaps using the Rosary in other prayers.
Or perhaps in holding the Rosary and meditating on Jesus death on the cross.
You are best to refer to an authorized book concerning indulgences or an authorized source of some kind. Other requirements are generally also needed such as going to confession, receiving communion, praying for the intentions of the Pope.
But remember this – earning indulgences are a temporal benefit. Celebrating the Mass has eternal significance and thus it is not given indulgences because the Mass – Jesus body and blood given for us – has far greater value. Sometimes I hold a Rosary in my hand during parts of the Mass (I am only a lay person but I sing the in choir). The Rosary gives me extra comfort. So it is even possible that the Rosary provides you and me with greater benefit than all possible plenary indulgences put together.
The Rosary is indispensible and of great value. Praying the Rosary, holding the Rosary, meditating on the mysteries of the Rosary, or using the Rosary beads with other prayers.