change the template dynamically

[devcrew]

I have an app installed named "conjoured referral" the website is for members only, I have some pages which are login protected and some are not. 
there is a redeem page for referred friend at https://ginger-daily.com/a/refer-a-friend/redeem/3ccaykjjigh9gsmsdw5okxf3ngvnlhjvbpy2dnan/5375 
a default theme.liquid template applied to it , which is login protected, and this page is created dynamically and not present in pages at backend. 

my question is that how I can change the template for this page dynamically? 

Thanks

[KetanKumar]

@devcrew 

sorry for this issue 

also sorry dynamic page template doesn't change its required to manual choose custom page template