Does Bear Umbra trigger if it was attached with Sovereigns of Lost Alara?
Bear Umbra's ability triggers when the creature it enchants is declared as an attacker. That happens at the same moment Sovereigns' ability triggers, so by definition it's too late for Bear Umbra. You have more luck next turn.

508.1. First, the active player declares attackers. This turn-based action doesn’t use the stack. To declare attackers, the active player follows the steps below, in order. ...


508.2. Second, the active player gets priority. (See rule 117, “Timing and Priority.”)


508.2b Any abilities that triggered on attackers being declared or that triggered during the process described in rules 508.1 are put onto the stack before the active player gets priority; the order in which they triggered doesn’t matter. (See rule 603, “Handling Triggered Abilities.”)