750Ti can mine Monero at a very low hashrate, but it is quite power efficient. Probably capable of some other algorithms too, although at low speeds.

If you want a new piece of hardware, just get a small x86 rackmount (with Atom or Celeron is okay) and use something like VyOS (https://vyos.io/) or PFSense (https://www.pfsense.org/) - it will be able to handle much more traffic than an 18xx ISR as well as many more features. Supermicro makes some good cheap barebones machines with Atom CPU in a quarter-depth 1U, about the same size as 18xx ISR.