Thanks for your post and welcome to our community.
I'm really sorry that you're having an issue with unwanted messages on your new SIM Only deal, I can appreciate you're keen to get this sorted.
Just to confirm, if the SIM is in a router how are you viewing the messages?
As you've taken out a SIM Only with us we'll have provided you with a mobile number. This number could have been recycled and may previously have had these messages sent or some companies try their luck and enter random numbers.
We can look at changing the number for free as it's the first time and see how it goes from there.
If that's something you want to do then please pop me a personal message