@Kngdv341 thanks for the screenshot of the settings.  I ended up returning the Deco router and getting a different TP link model which auto detected the EE settings on my sim and worked fine in minutes.  
 

I'll try your settings if it should ever fail though. 

  @Leeds_guy 

Hi mate,

Which TP link model did you end up getting? Having the same issue as you. 

Thanks

  @AcerSwift  I ended up with the TP-Link 5G AX3000 Wi-Fi 6 Telephony Router
(SKU: NX510V)

I bought it from a company called Transparent Communications, along with one of the mesh router units.  Worked perfectly straight from the box.  I've used it with 3 other sims since and they've all been detected correctly immediately too. 

  @Kngdv341  Hey there, im having the exact same issue with the same router. I have tried EVERYTHING you mentioned and still it didnt work. The ee sim works fine on my phone so its a problem with the router and the ee sim. Please help asap with any other solutions.

  @Lucs1 Hi.  I ended up buying a different router.  See my previous message for details.  I honestly think there's a software and hardware issue with the Deco, or more likely it's a dodgy batch?   Some users seem to have no issues with it at all.