well, limited for just ... how much, 1/70 of the population. I think limited is a very positive word in this case. I would never market there. Until the Internet situation will improve, anyway.
If you do online business with specific URL then you can try to boost traffic from worldwide then your sales may increase.
For conferences: Online methods: Try Facebook - create event But for Congo, like what the rest mentioned, focus on Offline methods like newspaper ads or even giving out flyers at business districts. All the best!
newspaper ads is really expensive as method. Probably flyers are much better, you just have to invest in time, first of all. Many of them will be trashed anyway very easily.
In online marketing you should engage with Social Media marketing. More Social Media followers means more prospect clients. Facebook, Twitter, YouTube, and Instagram are the best Social media that can help you promote your product, services and more.