9 months from brand new SE Model A2296 shows 4G but when calling becomes 3G

G'day!

Purchased an iPhone SE Model A2296. Mobile phone worked great but started acting up from yesterday. Top left screen always showed 4G. Calling Australian landline and mobile phones takes around 2 to 3 seconds delay before the first ringing on the other end. Video and audio calls were great with all calls all with clear voices on either side. Internet access were quick. YouTube access was clear and audio great and no latency of videos. Internet access were quick.

But from yesterday, 9 April, 2021 Friday, my SE acted strangely. Before l make a call, 4G is on top left screen, but as l make a call, 3G appears and some degradation of the voice can be noticed. But as soon as l hang-up the call, 4G reappears after a few seconds. YouTube, emails, video links embedded in emails works without issues. I had done resetting the network settings 3 times. I removed the SIM Card, wiped the SIM Card with a fiber cloth and reinstalled it. I still see the same problem, only when l make a call, the 4G changes my call to 3G and on a few occasions I had to repeat the same call because of the error connecting the call.

Have you any idea what is happening and how do l resolve these issues. I have attached 2 photos to prove my point. The first photo is just prior to my making the call. 4G clearly is there.

Second photo shows me in the middle of the actual call and now 3G appears and that small circling icon indicated its trying to connect to the vodafone network tower. A telecommunications tower is about a kilometer away from my home.

How do l resolve these issues. My problem could be that as l use my iphone SE outside the city outskirts, my calls may change to 3G and l may have real mobile phone or landline connection problems.

I need help in resetting the 3G status when l make a call and restore it continuously to automaticly stay as a 4G network without my changing any current settings.