I've been searching forums and the web and cannot find an answer regarding mobile security.

I recently started connecting my desktop to the internet through my mobile device's USB tethering and sometimes wifi hotspot.

I have UFW (uncomplicated firewall) configured and enabled on my desktop. I also run ClamAV.

What I am wondering is if first connecting the mobile device to a router will add another level of security? A router would add another firewall, correct?

As you probably can tell, I do not know much about security. My primary concern is doing banking and other critical online transactions across an over-the-air cell phone signal and the cell phone provider's network. Are my passwords and so forth reasonably secure?

Any thoughts or comments on Linux and / or mobile network security will be appreciated. Note: I am not really interested in going the VPN route. Thanks!

Once you connect to the internet, your system becomes less and less secure.
I'd think a hotspot versus some cable or satellite would be an equal risk.

Ad's on tv refer to using coffee shop wifi as hotspot risks.

I use my hotspot from time to time, generally when I'm having (extremely rare) connectivity issues with my ISP. I use the same security settings on my computer as when I use my router--firewall with all incoming ports closed and necessary outgoing ports open, AV, etc. I have never encountered any issues, but I don't think of it as either more or less secure than connecting through my router.

I did find an article that might help.

First of all:
You are talking about two very different things here.
There's plenty of online material about Wifi security. In a nutshell, never connect unencrypted, do not (let your device) constantly scan for access points, and if you don't (know if you can) trust the owner of the AP, don't use it. Which probably goes for 99.99% of all public APs.

There's less material about mobile internet security.
I don't think it makes much sense to put your mobile broadband through a router first, if you're the only one using it.
You have stated correctly that you can reproduce your router's functionality on your Linxu machine - security-wise that's mostly the firewall.

Beyond that, and on a consumer level, there's no difference once you're connected to the internet. It's out of your hands anyhow (but the same goes for your wired connection).
Use encryption, use strong passwords, maybe even 2FA.

Thanks for the comments folks. Pretty much confirmed what I suspected. I'll mark this thread "Solved."