This guide is focussed on the 827 but is applicable, almost verbatim, to the 1721 (remeber to use the appropriate interface names, since the 1721 is a modular router therefore the ATM interface can be ATM0/0 or ATM1/0, while the ethernet interface is Fastethernet 0/0).
Additionally you might wish to have a look at some tips on DSL config:
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...