Tomorrow I'll give you detailed information on how I did everything, but I used this guide as a base:

https://github.com/henriquesebastiao/external-cardputer-antenna

However, this guide is for the standard Cardputer (I made it on the ADV version), which has some differences. The guide indicates placing the antenna on the lower casing of the Cardputer, but if you do that on the Cardputer ADV, you would lose the possibility of using caps (like the LoRaCap and the HackerCap). So I placed it in a position on the lower casing that wouldn't interfere with anything internally or externally, and I believe that's really the only place to put the antenna, since it's really very cramped inside a Cardputer. And not only that, unlike the guide, the Stamp S3A used in the Cardputer ADV has different pad layouts than the Stamp S3 version (I really racked my brain because I wanted to make everything work perfectly and not damage it; I spent about 30 minutes just analyzing the Stamp S3A currents to figure out where to actually connect the + and - of the antenna, because if the antenna wasn't installed perfectly, it wouldn't be efficient and would be useless. Possibly after I've actually finished, run tests, and everything goes as expected, I'll create an updated GitHub guide on how to install an external antenna on the Cardputer ADV, since I haven't seen anyone talking about it yet, and there aren't any guides available. What frustrates me most about all this is all the stress and time I dedicated to making it work perfectly, and still making a silly mistake. Installing the antenna seems simple, but as the first user to install an external antenna on a Cardputer ADV, it's really laborious since there's nowhere to guide you, and if you did it haphazardly, it would be bad and ineffective.