Fatwa of Sheikh Abū Bilāl ḥafidhahullāh (summarised).

When it comes to giving lectures and so on, there are four types of masjids: 1⃣. Masjids of Ahlus-Sunnah. With respect to these types of masjids, there's no issue whatsoever in attending, lecturing, admonishing and conducting classes in these masjids. 2⃣. Masjids belonging to the common folks (’awām) who generally love khayr. These type of masjids allow everyone to give talks, because they are unable to distinguish between the different speakers. With regards to these types of masjids, we shouldn't leave it for the ḥizbiyyeen. Rather, we should visit them and advice the people therein in hope that they are affected by our lectures more than those of the other groups; because we invite to good based on clear evidences, and perhaps as a result, they invite us to come back more often. 3⃣. Masjids which are under the control of the hizbees, however, they have lost interest in this Masjid. It's okay to give occasional reminders on the go, but not official pre-planned lectures; this is something specific for the masjids of Ahlus-Sunnah. Official lectures in these types of masjids will revive the masjid only in the favour of the hizbees. 4⃣. Masjid belonging to hizbees who are stubborn upon their bid'ah. We should not visit these masjids at all; not for the occasional reminder, and certainly not for lectures. Source: https://t.me/madrasatuna/2516