@GraemeM_2909364 Firstly, even though you have unlimited data, you will not be able to use it while outside the EU; all sim usage is chargeable while somewhere like Mexico.....and it ain't cheap!! That means that you would need to add on credit to your account.
If you really do want to roam, you would need to get into the settings of the router to ensure that the APN was set to mob.asm.net (You can usually do that by typing something like 192.168.1.1. into the browser url bar - but it will vary by manufacturer).
That said, you seriously need to take a look at the Price Guide: https://sundaecdn.blob.core.windows.net/files/SMARTY_Price_Guide_V32.pdf
To roam in Mexico, as I mentioned you will need to firstly add credit to your account because all usage needs to be paid for. The rate for data roaming in Mexico is 10p per megabyte - so that is £102.40 per gigabyte !!!
You would probably be far better off either buying a local SIM card or else considering an E-SIM for your phone (if it will accept it) and then hot spotting devices to the phone.
