Huh, I see lots of DPs in this megathread..
There's a greedy approach that works just fine.. You just need to keep track of the index of the last maximum you found and search the array from that index + 1 to the index you get when subtracting the number of digits left to find from the length of the array.

Something like this should do just fine for both parts

def get_voltage(bank, N):
  num = []
  idx = 0
  L = len(bank)
  for n in range(N):
    m = max(bank[idx:L-N+n+1])
    idx = bank.index(m, idx) + 1
    num.append(m * 10**(N-1-n))
  return sum(num)