Looking for a 2bedrooms Apartment

Start by finding an airbnb accomodation and negotiating a cheaper longer-term stay rate while you look around for an apartment of your own:

https://www.airbnb.com/s/Barranquilla--Colombia

Walk around looking for signs in windows:  Apartamento en Arriendo, Se Alquila Apartamento, etc. and contact those at the phone number on the sign.  Use Locanto to see ads from many owners who may not require a fiador, especially if you negotiate to pay several months in advance:

https://barranquilla.locanto.com.co/

Under INMOBILIARIA look at the first two links, the first for apartments and the second for rooms.  Often people who rent out rooms will include laundry service and meals, mostly for an extra charge.  No fiador is ever required to rent a room, as far as I know.  And I can guarantee that you will be immersed much more quickly into the daily life of Colombia that way, and at a low cost.

Walk around some more and when you see a building that you like, if there is a portero or celador or vigilante or guardia (all words for doorman/security guard), talk to him and offer him a finder's fee, say, 100,000 COP (just a bit more than $30 USD) if he lets you know of an apartment for rent in that building, and if you end up renting it.  That $30 USD is almost 4 days' worth of minimum salary which is all many people make, so there's a good incentive for them to help you!

Finally you can also contact companies who act as fiadores, for a fee.  This internet search will get you several companies as well as links for apartments rented without a fiador as this is a problem for Colombians, too:

barranquilla fiador apartamento