How to file UK VAT?

I am a small-time EU seller, not storing anything in UK, but I have a UK VAT number as Amazon required it years ago. Now that Amazon's VAT services have ended I am struggling how to file UK VAT. Since Amazon is collecting the tax I should file "0.00" indefinitely, but I don't want to pay a tax agent for that.

I am unable to log in to the HMRC website. No matter what I do, their system says "We could not confirm your business". So, I am unable to file the VAT online. They also list hundreds of VAT-related softwares, but I could not know which one to choose. Can anybody recommend a simple and FREE software?

And do I actually need a UK VAT number anymore? Do I really need to keep filing "0.00" forever?