In 1914, laborers digging in the “terp” (artificial dwelling hill) of Tolsum, in the Netherlands, found a wooden writing-tablet. In 2009, it was deciphered, and it turned out to be a part of a loan-note, written in February 29 CE.
In other words, it dates from the winter after the Frisian Revolt (Tacitus, Annals, 4.72-74). Assuming that it was not brought to Tolsum on a later date, this means that the Romans regained or retained control of the Frisians. This is corroborated by the excavation in Velsen, which was attacked during the Frisian Revolt, remained Roman and saw a new building phase in 35. Probably, the Romans never lost control of the northern coastlands.
The text and a translation is now here.