It sounds like you're away from the physical table that everyone else is playing at. Which is why Roll20 is not going to do the job (I don't know why people keep recommending it).

So first off, Skype's audio codecs and bitrate are garbage. I would recommend trying Discord to see if it fixes your audio issue, and only use Skype for video. With that said, an external mic will always be better. I would personally recommend the Blue Snowball. I've been using it as a PC mic for a few years, and the audio quality is fantastic. Technically, it's designed to be more of a general recording device, and it has a switch on the back for omnidirectional recording. The one downside is that you will need to get a usb to lightning (or 30-pin, if it's an older ipad) adapter to plug it in. And obviously, you won't be able to charge the ipad while the mic is plugged in, so your friends will need to be sure to start each session with a full charge.