This walkthrough of Ableton Live's Link/Tempo/MIDI preferences tab points out a behavior that trips up a lot of producers: Link, Tempo Follower, and External Sync cannot run at the same time. Turn on Tempo Follower to sync Live's tempo to an outside source, and Link gets silently switched off.
Knowing this ahead of time saves troubleshooting time when Live isn't syncing the way you expect, especially in setups juggling a DAW, hardware, and other Link-enabled apps at once.
The same tab also covers the MIDI section, where you set up control surfaces, choose Takeover mode for hardware knobs and faders, and configure your MIDI ports.