I absolutely don't believe that what you bought would test to Cat7 requirements even when brand new. Cat7 is a proprietary standard, and as such is not terminated normally in RJ45 connectors, but in GG45 - backwards compatible with RJ45 receptacles, but looks more expensive (and is!). And you won't buy the tools to terminate it in a home improvement store. And you won't buy the cable flat. There's not much flat Cat7 cable sold by reputable professional cable vendors, and its uses are specific: multimedia in living spaces without running in the wall. It's not used to be pulled in any sort of conduit. It goes from a jack somewhere, then gets concealed (e.g. under carpet outside of walkable areas), and ends up in something in someone's living room, usually.
You had been had. Sorry to bring such bad news. What you wanted to buy (but didn't know) was Cat8 cable.
There's no splitting Cat7 nor Cat8 cables, pretty much. It's not possible to do in any sort of field conditions while still retaining full Cat7 or Cat8 performance. If done well it could perhaps "work" in that the endpoints were able to negotiate a 10G connection, but it'd be very, very iffy.
I would have had trouble doing even it in a well equipped electronics lab: in addition to a network analyzer (or a comparable instrument), I'd need an expensive test fixture allowing to connect each pair separately to the analyzer ($1k+ just for the fixture). I'd be laser welding the ends of the wire together under a stereo microscope, in a suitable fixture. No soldering anything, and the splice would be staggered, so that one pair each would be spliced every inch or two. That's to minimize crosstalk. Cat7 and Cat8 have quite stringent internal and external (alien) crosstalk specs. I've done it before without any testing other than the network equipment itself, and it worked fine, but it was hard to tell how close I got to the original performance. And it was done just to show that it could be done - beware of bets made over beer. I'd never use it for anything else.
So what you will do if you actually want 10G or 40G performance is as follows: use the old cable to pull a replacement through the same conduit, and forget about the whole Cat7 brouhaha. It's proprietary and you don't need it. And if it's cheap, someone is possibly lying to you.
If you're connecting within the building, then use either Cat8 cables if you truly need 10G-40G performance, or use Cat6e. I have had lots success with Monoprice Entergrade Cat8 patch cables. 50ft white will currently cost you about $50. There are other vendors, but you must inquire on homelab or similar forums about who is reputable. A random Amazon seller - not so much. If they are not in business of actually selling networking cable (and typically other networking gear) - forget about it.
If the underground connection is between buildings, then you're asking for trouble by using copper cable. And the trouble will come when you get a lightning strike close enough. You'll be pulling a replacement cable as well as replacing some gear. Not good.
Thus, for inter-building connections, you'll use your cut cable to pull one or two pairs of half-duplex LC-terminated single mode fiber patch cords (2 or 4 cords in total). Those are relatively cheap to interface with, but you're looking at about $300 worth of fiber and transceivers to get one 40G link up, with an extra two fibers dark as backup or future expansion. I'm assuming you have the network cards that support 40G SFP+ transceivers. If you want to be switching at those speeds - well, that gets expensive...
