Sump discharge freezing
How is a sump discharge normally set up in freezing climates?
Short and wide, with plenty of slope, so that ice cannot build up in the bottom of the pipe. A 2&quot; pipe to a 4&quot; pipe with a fall to a gutter or ditch leading away from the house.
Alternatively, completely below frost line to a dry well.
What can I do in the short term
Buy 350-450 feet of garden hose (or enough more than you already own to get there) and run water in the far end of the sump pipe. You'll have to keep the water in the hose flowing so it does not freeze up on you, and get it back inside a warm area when you stop.
Depending upon your builder, you might also try complaining that the extravagant mess they built (a 300+ foot underground sump drain fully qualifies for that description, IMHO) isn't working right, and you'd appreciate some help from them correcting it. This might clear up which pipe is which, if nothing else - you don't want to waste a lot of effort on the wrong pipe.
Contact a plumber with a pipe-thawing rig - the modern ones involve a pump that drives heated water into a smaller pipe inserted in the frozen pipe, which can normally be done from inside the house where power is easy to come by. Given the absurd length of your setup a portable generator or large heavy-duty extension cord may be required to work from the other end as well.
The town I grew up in had a portable boiler for thawing frozen culverts by blowing steam into them, but I don't think such a thing is easily found for rental. If you get back to long, heavy extension cord or portable generator I guess you could try a wallpaper or cleaning steamer.
What is the correct long term fix for this issue?
Hard to be sure without knowing where the problem actually is. If it's just frozen at the outlet end, an immediate red flag is &quot;the smaller pipe is the outlet&quot; and I'd start with digging that up, going back in at least 10 feet and replacing the exposed end of the pipe with a larger diameter pipe, ensuring that it is sloped a minimum of 1/4&quot; per foot. When burying that again, dig out 2 to 4 feet wide for 8-16 feet of pipe and lay a sheet or two of XPS insulation on top of the pipe, centered, which makes it act like it's buried deeper. Or apply more dirt and bury it deeper, which might mean building a retaining wall on the face where the pipe comes out. If the original pipe was not properly set, it may have lost slope with ground movement due to frost, causing water to pool and freeze.
If it's frozen in the middle of the run, that's a very expensive proposition to fix and is clearly your builder's fault for doing it wrong. Well, the whole mess is clearly your builder's fault for doing it wrong, it's only two years old.
There are electrical internal pipe thawing devices, usually intended for water supply lines, which can be placed and only powered as needed.