For use as just a modem, choose one from the ZyXEL VMGnnnn-B10x range and configure it appropriately.
I will always suggest the VMG1312-B10A but they are non-current ZyXEL devices and tend to be sold at a premium price (on eBay). Avoid the VMG1312-B10D.
Other member have different favourites from the ZyXEL VMGnnnn-B10x range.