Absolutely no reason for the solicitor not to accept the downloaded statements from the app, that you've printed off. They don't show anything different compared to the physical statements.you would receive.
I worked at NatWest / RBS customer service on the phones until 2018 and it was the case then that the only paper statements we could order were the transaction lists. It sounds like it hasn't changed.
I would be pointing the finger at the solicitor for being less than accommodating. No mortgage underwriter worth their salt is going to refuse bank statements printed from a pdf in this day and age.