can i configure remote capi on a Cisco 2811?
Which IOS is needed?
Configuration Guide available?
A cisco router can do all what you current product do, and much more. It does not need a server, however optionally it can use an industry-standard radius server for authentication.
The fact you have not found a configuration, is because you are not familair with the product.
To configure cisco productsI recommend you engage a reputable consultant or certfied cisco partner, and not dot try to do yourself.