John Chrysostom

See again Paul judaizing. "Where it was thought," it says, both from the time and from the place, "that prayer would be." Out of the city, by a river side: for it is not to be supposed that they prayed only where there was a synagogue; they also prayed out of synagogue, but then for this purpose they set apart, as it were, a certain place, because as Jews they were more corporeal - and, "on the sabbath-day," when it was likely that a multitude would come together. …

Homily on Acts 35 ↗

Theophylact of Ohrid

There, because of the small number of Jews, there was no synagogue, and the especially devout among them gathered secretly outside the city "by the river." As a more carnal people, the Jews, where there was no synagogue, prayed even outside of one, designating some particular place for this — they also prayed on Sabbaths, when the people usually gathered as well.

Commentary on Acts ↗

Oecumenius

Not only where the synagogue was, but also outside they prayed, as if separating some particular place for this purpose, namely the Jews being excessively given to bodily things. Or rather, perhaps there was no synagogue there because of the thin population; beside the river outside the city the supposedly pious used to meet secretly. [CHRYSOSTOM]

Commentary on Acts ↗