You say you use 3BB and then write that 3BB say there is no service to the condo?
Anyway, if there is no 3BB service to the building/condo then 3BB need to do some work with manpower and materials to provide a service for you, fiber optic cable, terminal unit and router. You need to check if the condo unit will allow 3BB to install the system.
Are you sure the 1500 baht is not the deposit and 1st month of service. We had to pay in advance such amount as a sort of guarantee to start the installation and the service is for a 1 year contract so they can recover their cost through the 1 year of service. If you stop the service earlier than 1 year there will also be a penalty fee to pay of about 4500 baht.
Depends what you want but you can also buy a 4G router, subscribe to a 4G internet package.
Is this in Bangkok?