My son has a Samsung A3 phone we gave him which we loaded with a Ultimate Oomph Unlimited SIM. It's his first ever phone and he's been using it for months without me realising that he can only connect to the internet, download apps, get WhatsApp messages etc when he has wifi but not when he is connected to mobile data (you can text and phone though). I've googled it to find a solution but can't seem to get anywhere - yes his mobile data is switched on.
We're using Family Link and I don't know if that is making a difference. The phone was previously used with another SIM card so don't know if this could be the problem - read that APN Settings could be an issue and so as instructed on the website we sent a text message with the word ALL to 789922 but the phone still doesn't connect to mobile data.