Are you serious? These are 20+ year old remotes. I'm sorry to be the bearer of bad news but electronics don't last forever. As great as URC hardware is (especially the old stuff, which seem to last forever), I'm really surprised those remotes are still going at all (if they were daily drivers).

By analyzing what you said, you have 1 remote that still connects to the software fine. Same cable, same computer, and the other 4 will not connect anymore. Seems to be obvious the 4 remotes are done unfortunately.
I don't think those models had a hard reset you could do, that might allow them to connect again. But doing a deep search on this forum board might revel something. The forum archives go back a long way.

I understand the frustration however. Upgrading to something newer (like MX890 or MX990) will mean getting the software to be able to program it yourself. You need to make 100% sure that if you go that route, the place you buy the remotes from 'will' supply you with the needed software. Or you could be screwed.
Also be warned: time will tell but I wouldn't expect any remote from this decade to go anywhere near the distance those old MX700s did. They just don't build things like they use to... no body does :(