I wish I had some "magic" to lay on you, but I don't, for some reason OS wireless units are notoriously flaky. I decided today was the day to put new bats in the anemometer and the rain gauge as the rainy season will start in a couple weeks. I replaced batteries in both, reset both and came back down to check results. I have both a 200 and a 100, the 100 had picked up the sensors already, the 200 needed to "rescan" the sensors, but picked up both units.
About a year ago my rain gauge failed just like yours during a battery change, I replaced it. I see by your note on the 8 micro screws you have the old design, the new one has a tab release - - - good grief, as I typed that I just realized I didn't put the battery case door back on - oh boy, thought I was done for the day, oh well, what's one more trip to the roof, I've almost lived up there this week putting in a new sat dish, aligning it "manually, alone" etc.
Try another reset and then have the 200 scan for sensors and if that fails, try totally powering down the 200 for 10-15 minutes and then reset everything and see if it picks it up.
Good luck, I'm headed for the roof - darn!