Ring Virgin. And Vodafone Paul. They TOLD me it was a roll of the dice 🎲. Whether your phone connected to 5gh and how much range etc . This phone has very little! I was so confused myself! As much 'old ' samsung had no problem at all! Still doesn't! It's a nightmare 😫
Simple. Log in to router settings and check both 2.4 and 5 are enabled as your A12 only uses 2.4 you won't have to worry about it switching bands but do this for any other devices you have that may support both. Next position router as well as possible as advised prior. In A12 WiFi setting 'forget the network' and rejoin as doing it for first time. In same room a router one would expect full WiFi signal. If have other devices check how these on 2.4 perform. If only A12 having issues try on another person's WiFi if still issue reset A12, if still issue contact Vodafone as 24mnth warranty. If others devices have issues with your router also then reset, if still same then contact Virgin.