You can now get quick links.for Shimano.Just Google Shimano Quick Link.I don’t bother I always use KMC chains on myUltegra mechKMC X10 or sram pc1031 but they ar both a little more on Amazon but you get a powerlink rather than the Shimano join pin, unless they've changed their method.
Original chainThere's a good chance that the Shimano CN-HG54 chain 'snapped' at the link the OP (or the bike shop that replaced the chain) joined. Either that or the OP's immense power and/or gear canging expertise. The KMC or SRAM 'quick link' joining method seems less risky (and I, too, use and would recommend one of those two makes).
After "1800 miles poor maintenance" a new chain will very likely slip on the current cassette so I recommend the OP order a new cassette as well, immediately, unless they have one in their spares box waiting.