I'm working on a community wifi project for an impoverished settlement. We have a router and a cable modem installed in a container house, sometimes without heating for days. Both are designed for indoor use. (Wifi coverage is provided by outdoor APs.) What should I do to protect the indoor equipment from frost? Outdoor temperatures sometimes reach -15 °C (5 °F), the container is made of metal. We would need the cheapest workable and safe solution. The cable modem is provided by the ISP and they offer no outdoor equipment (but we may be able to convince them to let us use our own one).
Whenever I am deploying something outside in an enclosure, this is the procedure I use. First, make sure you start with the proper enclosure (for example NEMA 3 or 4). If you penetrate the enclosure, make sure the penetrations are properly sealed to maintain the rating. (I assume this would be the case as you need electricity and because you mention that the enclosure is made of metal and no one wants their antennas inside a metal box, right?)
Since your enclosure is going to be restricting air from flowing into/out of the enclosure, you now only need to contend with any moisture that is trapped within the enclosure with your devices.
Your next step is to reduce the moisture in the trapped air. Low moisture means less chance of condensation and with no condensation, you get no frost. For this, I prefer to use silica packs. You can buy these cheaply, but even better they come in the packaging with many items (shoes, clothes, electronics, and so on). Most people throw them away, but I recommend saving them as they have many uses.
Before you close you enclosure, add some silica packs. However, before you do that, make sure the packs are "fresh" by heating them. Heating them (120-150C/250-300F) will cause them to "release" the moisture they have gathered. For this, I use either an oven (longer, but you can easily set the temperature) or microwave on 1-2 minute cycles (faster, but you need to figure out a way to determine if you have heated them enough and some outside packages won't do as well).
Now, silica packs will not remove all the moisture from the air, but you don't really need to remove all the moisture. The waste heat from your electronic devices will help to prevent the moisture from condensing on them and most of the condensation (if you have any) would be on the inside of the enclosure itself (which will tend to be colder). Make sure you account for this by not setting any electronics or power cords directly on the bottom of the enclosure.
Buy an inexpensive cooler -- a big plastic or styrofoam one like you'd use for a picnic -- remove the lid, place it open side down over the equipment. As long as there's no drafts in the container/house-unit, the heat from the equipment will stay up inside the cooler. If it's really cold, move the wall-wart power supply/ies inside the cooler too.