The answer is it's possible, but depends on the ISP.
ONT have a setting similar to cable modems that specify the maximum number of devices that are allowed to be bridged.
Some ISP is only set this for one, some ISPs set this for two, mainly to allow for a customer to swap routers and not have an issue.
The best answer is to try it and see what happens.
You're not going to break anything.
Use SFTP or FTPS, plain FTP should not be used.