Funds for your Coin MF purchases are debited from your trading account. Your trading account is directly linked to your bank account, so to buy MFs on Coin, all you have to do is transfer money to your trading account from your bank. If you do a NEFT/RTGS/IMPS from your bank, then there are no transaction costs from our end.
If you find it a hassle to keep track of when you have to transfer funds to the trading account to ensure you don’t miss your MF SIPs, you can issue a standing instruction from your bank account to your trading account for the days you require to have funds for your MF purchases. The steps for this are mentioned on the link below:
Also, this is an excerpt from Nithin’s post on why you should buy Direct mutual funds on Coin held in Demat form.
The entire post is here, do read:)