You do know that mobile data and cellular service are different things right?
You can still call and send sms without mobile data, which is why it still shows you the bars. Turn it into airplane mode if you want to turn the modem completely off for some reason.
Oh i understand what you mean now.
Step 1 : settings connections wifi, click the 3 dots, go to intelligent wifi. You have to be connected to wifi for the next step, then you have a “switch to mobile data” setting inside the intelligence wifi, turn it off.
Step 2: settings developer options, “mobile data always active”, turn it off.
I forgot that these 2 settings are on by default as i always turn them off day 1, my bad.