£20 is extremely inexpensive for a mobile network modem. Other providers, I don't know what terms you agreed to with Orange, either give you a free mobile network modem that's capable of speeds of up to 3.6Mbps with an 18 month contract at something like £25 per month, or charge you £50 for the modem with a 12 month contract. For the faster modem capable of speeds of up to 7.2Mbps they charge about £50 and £25/month on an 18 month contract or about £100 and £25/month on a 12 month contract.
So if you bought the mobile network modem at £20 and no monthly charge, then it's no wonder you received such a bill if you have been surfing away. You could argue with Orange that they didn't explain this at the time.