In my opinion, there are two factors that are working against Ronaldo right now.
One of them is his age as the guy’s 37 right now and his body can only do so much for himself at this top level of football. As a matter of fact, the Premier League is an extremely physical football league right now, so playing there at the age of 37 and expecting the best out of him is a no-go.
Another factor could be the fact that he would like to play in the Champions League but is unable to do so. As a result of the fact that he wanted to leave United because he wanted to play in the Champions League and now he is not getting the opportunity to play, his morale has dipped and is affecting his performance.
Regarding the first factor, the age problem which has reached 37 years is indeed old enough for a football player. but ronaldo's stamina is still above average because of his healthy lifestyle and good at keeping his body proportions. but it can't be denied anymore, the problem of age also results in a decrease in body condition that can't be avoided because Ronaldo is not a robot.
and the second factor about him continuing to want to play in the champions league is certainly the reason ronaldo rejects many clubs from outside the champions league. but some football clubs that play in the champions league do not dare to bid on ronaldo because the salary that ronaldo pegs is still very expensive. that is the main reason why he is still at Man United, even though Ronaldo doesn't want to be at the Red Devils club. ronaldo is in a dilemma about his future and where to play.