The built in antenna's/cards are still relevant. Nobody likes to have an antenna tehy need to carry with their laptop. However if you are using just an antenna, then I would look for a replacement internal antenna for your wireless card. However it sounds like you are using an entirely new wireless card connected externally, most likely via USB. Since this works as you want it to it sounds like your internal wireless card broke. This can occur for many reasons (such as a drop, heat source, power surge etc).
To fix this, first look up the PROPER way to access the wireless card for your specific computer. Second, ensure that the antenna is connected to this car properly. Reassemble and check your wireless. If the problem still occurs, order a wireless card that is compatible with your model of computer. Preferably the same part that your computer is already using (check your warranty! This may be free!). Once you receive the part you need to remove the old one and put in the new one. Properly. You should be able to use the existing antenna that is built in as they do not break often (but are cheaper to replace, hence me stating to check it first).
Note: This will resolve the problem 99% of the time, but like anything to do with computers there are other issues that could cause this problem.
-- Windows 7 Ultimate on custom built system, Windows 10 on under powered laptop. Sophos UTM 9, Ubuntu Server and Windows Server 2008 R2. HyperV Virtualization --
"The hottest places in hell are reserved for those who in a period of moral crisis maintain their neutrality," John F. Kennedy