You are correct that you probably don't really need a VPN in that setup: connect the iphone to cellular data, set up a hotspot on it, connect laptop to the hotspot. Do banking via HTTPS secure web sessions on the laptop, connecting to your known bank website directly. ETA: don't go to malicious sites ;-)
Particularly since you are wanting to do banking, if you want to go with "trusted" name brands, here are a couple options, and they offer monthly plans as well as annual:
Norton Secure VPN:
https://us.norton.com/products/norton-secure-vpn
( see review at
https://www.pcmag.com/review/354003/norton-wifi-privacy-vpn )
McAfee Safe Connect VPN Proxy App:
https://www.mcafee.com/consumer/en-us/store/m0/catalog/msc_558/mcafee-safe-connect.html
Note the pros and cons re: where they are located etc which may or may not be a factor for your needs.
SW