Going by the ideology/doctrine of the ahmadiyyah movement, as we were told they believe another prophet will come after Muhammad(s.a.w), is it permissible to observe solat in their congregation?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Whoever believes that a Prophet will be sent after our Prophet Muhammad, sallallaahu ‘alayhi wa sallam, has disbelieved in Allaah and denied the Quran, as Allaah says (what means): {Muhammad is not the father of [any] one of your men, but [he is] the Messenger of Allaah and last of the prophets.} [Quran 33:40]

All the Muslims agreed in a consensus that there is no prophet after Muhammad, sallallaahu ‘alayhi wa sallam. Therefore, if anyone claims that someone will be sent as a prophet after Prophet Muhammad, sallallaahu ‘alayhi wa sallam, then he is a disbeliever (non-Muslim) and it is not permissible to pray behind him or to eat the animals slaughtered by him, and we do not engage in marriage relations with him, and we do not say Salaam to him, and he is not buried in the graveyards of the Muslims. Rather, all the rulings of non-Muslims apply to him until he repents and embraces Islam and believes that there is no prophet after the Prophet Muhammad, sallallaahu ‘alayhi wa sallam.

For more benefit, please refer to Fataawa 13402 and 88413.

Allaah Knows best.