If it were a case where you had the Asus upfront of everything else, then I'd say replace it. But behind the AT&T modem/router (which I don't think is a true bridge) doesn't make much difference in that case.
Before you expose anything directly you should make sure that it's been updated for known vulnerabilities.
Synology : Security vulnerabilities
Same would apply to the Asus. It's had issues of its own.
Don't know whether there's any significant performance advantage to one or the other. Looking quickly I do see some complaints about the Synology VPN being slow but didn't dig in to see if that's user error, config, whatever. Might take a deeper look. Also check the client setup to make sure that works well on whatever you'd be using to access it. I'd assume that Synology is good as far as common things go but never know what and what versions, etc., somebody might be using.