I have downloaded a few free V5103 service tools. None of them worked. Each time an operation was attempted the response was error 002, meaning a function not supported by the printer was attempted. Luckily no harm was done to the printer. Nothing was wrong with the printer, other service tools worked as intended.
The use of a too old service tool (too low version number) will block access to service mode. I have a suspicion that a higher version number will be needed for your printer.
It has been recommended to buy the newer service tools via eBay, as you can use PayPal so the seller doesn't get your credit card info. But to my surprise it looks like all the offers on eBay of Canon service tool have mysteriously ended, sold out, are out of stock or similar. I suspect Canon has something to do with this.
Here is a download of v5103 and
here is the much newer v6310. Note that it looks like the price has been changed probably increased. It looks like you can use PayPal so you only risk the cost of the service tool.
Be very careful with which site you buy a download from. If you are lucky you get a functional service tool. If you are unlucky your credit card info might be offered for sale on the dark web.
There is an alternative to the Canon service tools.
It is the WIC reset program that supports a lot of Epson printers and also some Canon printers. It supports the TS700 series of printers which I think means the TS704a is also supported. It is free to download and use to get some info from, but to do a reset you need to buy a reset key at $10.