Most bike phone holders are adjustable to your phone's screen size. Redmi 9c is 6.6''.
Check out one of these:
https://www.skroutz.gr/s/31196068/Ugreen-LP494-60548-Bicycle-Mobile-Phone-Holder-4-7-6-8-Black.html
https://www.skroutz.gr/s/24850769/Hoco-CA73-HC-CA73-Bicycle-Mobile-Phone-Holder-4-5-7-Black-Red.html
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