Why harbour torpedo nets could not solve the Dardanelles mine threat

Could the pre-dreadnoughts' existing torpedo nets have protected them from Dardanelles mines?

No. The nets worked while stationary, whereas movement and larger mine charges required different cables and supporting structures.

Pre-dreadnoughts could deploy torpedo nets while stationary in harbour. Underway, water resistance would push the nets out of their intended position. The mines being discussed also carried larger explosive charges than contemporary torpedoes. Protection for an advancing ship therefore needed different cables and metal framing suited to movement through the water.