PIP back pay

Basically phoned DWP on 65 birthday 2015 and said I am on dla can I claim mobillity when I return to uk and they said yes but that's all just yes.Came back to uk in 2018 tried to claim mobillity and they said no.So because It looked like they were calling me a liar,kept asking and asking.then in Jan 2020 without me asking said I had applied for PIP and gave it me with mobillity.I had been trying to claim it on dla but after 65 u cant get it.But thats what the girl meant in 2015 but she did not mention PIP just yes u can claim mobillity but they must have known what there original adviser meant but did not elaborate the bxxxxx