The driver that Apple includes for FTDI-based serial devices appears to be broken in High Sierra (verified as sill broken as of 10.13.3). We have found a workaround which we describe in a blog post. Use this topic to let us know if it works for you and to ask questions.
Update April 17, 2020
Thanks to some great sleuthing by a forum user, we've discovered that there is also a PowerLinc Modem version component to this issue. He had an older PowerLinc which identified itself as v2.1. When he upgraded to Catalina, he experienced the issue. After attempting all of the various workarounds (drivers, hubs in various configurations, etc), he decided to buy a new PowerLinc, which identified itself as V2.5. He attached it directly to a USB3 port on a MacBook Air, and it worked.
It's unclear if it's some hardware change or something in the firmware (or both), but it does appear that this also plays a role in the issue. So, a reasonable attempt to overcome the issue is to get a new PowerLinc and give it a try. If it doesn't work you can always return it.