You can use VPN but it needs to be for same region. If different region, causes confusion and problems.
I know it sounds like most everything else is fine. It’s possible you reached maximum amount of views for some ads, but it’s best to submit a Support Ticket just so Brave can look and make sure no issues. You can create a ticket by completing the form at https://support.brave.com/hc/en-us/requests/new?ticket_form_id=360001302431
On a side note, when I mentioned possibility of you reaching ad cap, I don’t know if you’re aware of those limits. If you never noticed, might want to also check out Understanding Brave Ads Catalog Viewer as it will link to site to see what campaigns are in your area, which devices they show on, the maximum amount of times you can see, etc. If you notice low campaign amount and/or not many available for your region, that also can explain why lower amount of earnings. I’m only sharing this as extra thought just in case it might apply, otherwise it’s just extra information. Regardless, send a ticket and hope Brave can look soon.