which fastag is connected to bank account? I think none! At least not from the big banks like icici, axis, hdfc, sbi... All will have dedicated wallet and no where related the bank account. it needs separate recharge like paytm wallet. Only difference in paytm tag is, it is already linked with the main wallet, no separate wallet for fastag specific.
I dont think you can topup the fasttag balance, they just keep 150 aside for it from your paytm wallet balance which can only be used for toll payment if you are low on balance in your wallet but have to pay for a toll. Also your fasttag will be blacklisted if it doesnt have the minimum 150 balance reserved for it,so you can use that reserved 150 to pay for toll once but then it will get blacklisted. Someone correct me if i am wrong. at least this is how I think airtel fashtag works.