Telegram offers unlimited disk space for storing channel files for free.
Yes, some channels have ads, but they're in the form of separate messages that accompany the content — you can just scroll through them. Many don't have ads at all.
Of course, using a temporary number is preferable for such purposes, but I'm not sure how Telegram would react to that - probably have some extensive blacklist of networks and operators. In my country, SIM cards are sold by the pound and don't require any documents, so I simply bought one, activated my account, and threw it away when I was in another city by work.