My website is using a mobile-only theme. But a problem arises.

My website is using a mobile-only theme. But a problem arises. Responsive theme, which is the PC version, is applied even if you connect via mobile. When I asked the theme seller, they said that it was a cache problem, so I could delete it, but I couldn't delete it and they did not give me a proper answer. Even if I access my website through mobile, the mobile theme does not work. Does anyone know how to solve this problem?

I'm not good at English, so I'll translate it and organize it again.

Problem 1. I am using a mobile-only theme, but when I access the website on a mobile device, I am connected to the PC version, not the mobile theme.

2. The theme seller told me to delete the cache, but the problem is the same.