If someone accepted shabbos early (tosafot shabbat), and remembered he didn't put on tefillin that day, may he put on Tefilin in order not to miss a day of performing that mitzva, or is he already bound by the prohibition of putting on Tefilin on Shabbos?
According to Achronim in the end of Siman 30 they discuss on a regular day of the week if one already Davend Ma'ariv he would still put on Tefilin, in Mishna Berurah 600:7 he mentions about a scenario of a congregation who did not blow the Shofar on Rosh Hashana until after they were Mekabel Shabbos - he says that they blow without a Beracha, accordingly one would put on Tefilin in the scenario in question.
The Sefer Avnei Yashfei 8:54:1 writes that if one forgot to put on teffilin and was mekabel shabbas after plag and even made kiddush(actual story happened) he can put on teffilin since he accepted shabbas by mistake and would not forgo his mitzva of teffilin if he remembered. He brings a few sources for support,the Taz 600:2,The Shoel U'Mashiv 2:23 and it is also brought in the Mahrsham(Daas Torah 261:1).
He brings The Shevet Hakhasei 5:49 who holds one should put on teffilin but without the bracha,this is also mashmah from the Taz(600:2)
He also held that the bachur had to make kiddush a second time,since when he put on teffilin it shows he said it by mistake and it really was not shabbas for him yet(if one would ask about a bracha levatalh ,he talks about it later on in the tshuva)