This may not be the problem nor solution - I debugged this *endlessly* with my GEM45, and a powered hub didn't help (but I did spend money!).
See the culmination of my mostly-complete "saga"
here
.
The USB to RJ12 adapter however did *not* work, and the guiding issues were because I suck at polar alignment.
I did run into the "[ERROR] Failed to connect to port (/dev/ttyUSB0). Error: Port failure Error: No such file or directory. Check if device is connected to this port." problem as well - it was because the Pi couldn't assign it a USB adapter correctly, iirc because of the mount itself (was often the problem with the GEM45 when going direct-to-mount with the Pi).
You can see this on the Pi4 terminal by running:
dmesg -Tw
Then plug in the mount however you are doing this via USB - you can see what that will likely look like
here
if you are experiencing the same problem I was.
TL;DR: Grab a cheap, *unpowered* USB hub (I linked directly to the one I ended up using) and try that - it may save you a massive amount of headache / money-spending.