Had knee surgery 5 months ago for tibial plateau fracture, and my lateral ligament is still weak. any recommendations?

Tough to answer: The majority of tibial plateau fractures do not involve the lateral ligamentous structures. In general, the lateral structures (fibular collateral, popliteofibular, popliteus, etc) require surgery to repair or reconstruct. I think your case is much better addressed with a specialist and careful review of the situation.

Lateral ligament: I would get an MRI to check the status of the ligament and also to make sure that the plateau fracture is adequately reduced.