Test router DSL3782

Hi. 

I have tested the D link router for 3 weeks now and i have come to the conclusion that it has solved the problem with my fibre broadband speed problem i was experiencing with the Huawei. The test router came with a letter saying i had 21 days to test it and after that i have to send it back otherwise i would be charged for this router. I dont want to be charged,So,what should i do? 

Thanks