Talk Mobile data problem

Seeking any suggestions on how to solve this.

I have a OnePlus 12 (UK) and am on Talk Mobile (ie Vodafone). There is a problem in that I keep getting a notification that I ‘have no internet and might be out of data’. Thing is I have plenty of data! Also most apps seem to be ok (except BBC News most notably) and the issue arises when using a browser like Chrome or DuckDuckGo. I cannot reach any website.

My Talkmobile is SIM 1. When I switch to SIM 2, which is spusu (EE) the problem goes away.

The problem is exactly as described in another forum by someone else (no answers):

Question - "No Internet - You may be out of data" notification and data not working properly - weird SIM slot behaviour (CPH2415) | XDA Forums

Can anyone help? Thanks!

I have also posted this in the OnePlus sub.