Falsely accused of tampering with the product under guarantee...

I bought an UPS from ACDC Express Gezina on 2022-07-05 It stop to function properly on the Friday before 2022-10-17, when I took it back for warrantee repair. The Unti stayed with them for 3 week even after I ask nicely that I need it urgently back as I use it for work. I repeatedly phoned and promises was made to phone back which never happened. Then when I arrived there on Friday, It was said that the unit was already there more than a week ago, nobody informed me. On inspection, they did not do anything as they accused me of tampering with the system. I did NO SUCH THING, as I was using it. The unit was exactly the way I received it. So it seems to be that they have either sold me a second hand unit for new or someone in the pre-delivery has tampered with it...They refuse to do any corrective action.