Zwave range is fixed by standard, it shouldn’t really vary between one device and another. Except that zwave plus devices do have significantly longer range then Z wave.
I’m wondering if you may need to do a zwave repair on your network. That rebuilds all the address tables and makes everything run much more efficiently.
It sounds like a lot more work than it is. You don’t have to fiddle with any of the individual devices. It’s just a utility that you can run right from the mobile app.
I will include the link to the FAQ on range at the end of this post.
Meanwhile, to answer your question, if the lightbulb is A zwave bulb, such as the ones from linear or Aeotec, then yes, it will act as Zwave repeater. However, these devices are not usually rated for outdoor use. Sometimes you can bounce a signal through a window, or there may be a sheltered porch area where the bulb will be OK.
If the lightbulb is a zigbee light link lightbulb, like a Phillips hue, it will not repeat for devices other than other ZLL lightbulbs.
So if you do want to try a lightbulb, make sure it’s the right kind.
You should also get in touch with support@smartthings.com. They can look at some diagnostics on their side and see what’s going on. It could even be that the fountain was just not fully excluded from your previous network.
Here’s the range FAQ. This is a clickable link: