Yes. Plenty of couples plan their Ugandan wedding from overseas, and The Aisle works from anywhere with an internet connection.
You can browse vendors, save favourites, and message them exactly as you would from home. It helps to tell a vendor early that you are planning from abroad and to share your wedding date, so they can work around the time difference and let you know how they handle deposits. A deposit is money paid in advance to hold your date.
If you would like someone on the ground to run things for you, look for a wedding planner on the Find A Vendor page. Many are used to working with couples overseas and can be your eyes and ears in Uganda.