L3 License is dedicated for CPE not WISP
SXT has a narrow beamwidth antenna. You simply need to buy the right hardware. License will not help.
If you only need to connect your two units together, then use bridge mode instead of ap-bridge. The former is limited to a single associated client, but they are the same otherwise (read more here).
In case you need to connect more than one client, however, you should buy proper hardware. The cost of L4 license will be comparable with the cost of a proper hardware unit with L4 license already. Read more here.