Why does my mobile data work only on hotspot?

I own a Samsung Galaxy A71 and I’m on Reliance Jio network.

When I had bought this phone and put a new Jio SIM on it, my mobile data worked fine, but the hotspot didn’t. My laptop browser said ‘DNS not found’ or something along those lines. I solved it by updating APN settings.

Fast forward to eight months.
Right now it’s the exact opposite. My mobile data works very rarely. I get maybe a bar of signal at best. On 4G network I get 20-30 Kbps. A WhatsApp message goes through after 10 min. But if I connect my laptop through the hotspot, suddenly the data works seamlessly without any problems. A YouTube video on the laptop at 720p can load without any buffers, the same cannot be said on the app in my phone.

Any idea why this would happen? Something to do with the APN? Or something with the network?
Also, the JIO tower is right next to my house. Why would I get only a bar of signal?