Why not mount a cheap OpenWRT device inside a rugged IP68 rated case?
Inexpensive small-form factor devices such as the OpenWRT proven TP-LINK WR703N or TL3020, provide all the functionality you ask apart from the 3G connection > but you can fit a USB 3G or faster 4G dongle on the USB port. Inside an IP68 rated [plastic] box, a router is impervious to even salt spray. Obvioulsy power supply is through waterproof glands.
I'm not endorsing any of the products here, but you'll get the idea of the rugged enclosures available : https://www.data-alliance.net/enclosures/
edit + Tip: Remove a router from it's plastic case and mount the PCB inside the enclosure - this improves heat dissipation. To ruggedize further, solder in the power connection.