I purchased a GL iNet Opal router so that I can use free wifi’s at hotels and coffee shops. I noticed that the admin page has a submenu - Firewall. Since the router has a built-in firewall then can I safely log into my bank account on public wifi’s?
Not because of the router.
Your bank most probably uses TLS. The most an eavesdropper can understand is which bank is your bank. The same will happen if you use Opal without a VPN.
If you want an attacker not to know which bank is yours, configure a VPN in Opal, and make sure all your traffic goes through it.
What security does the Opal router provide when using public wifi compared to directly logging in?
Without using a vpn? You can hide which device you’re using.
You can use DNS-over-TLS/HTTPS to hide your DNS requests and avoid spoofing. Some DNS providers , like nextdns.io, allow you to filter out malware, trackers, etc.