Nice looking gear.
When you’ve got used to it and had time to fully asses it I hope you’ll post a review in one of out “Equipment” topics:
https://forum.audacityteam.org/t/recording-equipment-for-voice/17203/1
https://forum.audacityteam.org/t/sound-card-reviews/8375/1
The microphone is a mono device so it is expected that it will produce mono recordings.
What you need to know is how to make a mono recording that plays through both speakers (like when you play a mono radio broadcast through a stereo radio).
The way to do that is to plug the microphone into channel 1 of the Tascam US-800 and set Audacity to record “1 channel (mono)” in the Device Toolbar.