SDP help needed

I am I right in thinking that if we informed DWP of a change of address back in 2018 and he met the criteria then surely he should have had it paid from that date. I was not aware of what an is10 form was and surely when they processed the change of address they should have taken into account any changes that would have meant they reassessed his benefit claim and entitlement. I think it was the tone of the letter that had me mad, “ why has it taken 7 years for you to apply”. Because we were not aware he was entitled to it after reporting the change of address and going from the information what the person who took the call back in 2018 gave us from the questions she asked. Do we actually even stand a chance of even getting anything back as my mind is fit to burst.

Hi thanks for the link it won't let me go further than putting my postcode in. I am up against the clock and just don't know how to dispute the decision. How do they expect you to ask for a benefit to be applied or awarded if you are not aware of its existence. Would I be able to argue that when we informed them of his change of circumstances and address changed then they should of made me aware of this is10 form. I need to get the letter sent off in the next week and have spent 1hour 30 on the phone today to try and speak to someone to confirm where I send my MR to but it cut off at 6pm.