Hint : Take input as a string , map each of the digits , example '0'-> 0 , '9'-> 9 etc. simply take sum and solve the problem using recursion.

Example: sum of digits python recursive

digit_sum = lambda s: sum(int(digit) for digit in str(s)) #without recursion

#sum of digits using recursion

dsum = 0 # we define dsum outside of the function so its value isn't reset every time the function gets called recursivley

def rdigit_sum(s):
    global dsum # making dsum 'global' allows us to use it a function
    if s: # checks if s has digits to add to dsum
        dsum += s%10 # adds the current units digit to dsum
        s = s//10 # removes the current units digit
    else: # if there are no digits left
        s = dsum  # this block reassigns s to dsum, then resets dsum to 0 so dsum doesn't already have a value if this function is called more than once in a program
        dsum = 0
        return s  
    return rdigit_sum(s) # this is the 'recursive' part of the program that calls the function again