If the person wants to get a plenary indulgence for praying the Rosary alone in his home, all he needs to do is pray for the Pope’s specified intentions regarding the Coronavirus. He could also get a plenary for saying the Divine Mercy for the same intentions, alone in his home. There is no need to “wait” for anything, the Vatican decreed these plenaries way back in March, probably because the Pope could see that everybody in Italy and everywhere else was going to be stuck in their houses for extended times.
As I said, I also believe joining with a live group on Zoom or by phone or similar live interactive platform is a “group” for indulgence purposes because the Manual of Indulgences gives indulgences for joining our prayers with certain live broadcasts and receiving certain live broadcast blessings. But the Vatican decree regarding Coronavirus intentions makes that a moot point. It is also fine to pray for the coronavirus intentions and also add other intentions of your own, it doesn’t affect the plenary.
Finally, I note that the plenary indulgence is a bonus given by the Church to encourage people to pray, do works of mercy etc. It has absolutely NOTHING to do with either the efficacy of prayer or whether Jesus is present because two or more “gather”. Sincere prayer is ALWAYS efficacious regardless of whether you meet the criteria for any indulgence at all.