Neptune 4 Pro won't connect to my WiFi after editing the network config

Skip the file editing entirely, that's your problem. The Neptune 4 runs a Linux system under the hood and the network side gets handled by its own network manager, so a config file you edit by hand either gets ignored or gets written back over. I went thru the exact same loop when I set mine up, edited the file, rebooted, nothing. What actually works is getting in over SSH and letting the printer's own tools do it. Once you're in over SSH, type sudo nmtui and hit enter. You get a little menu right in the terminal. Pick 'Activate a connection', find your network in the list, enter the password when it asks, done. Took me maybe five minutes total after fighting with config files for an hour. The exclamation mark in your password is not the issue, it goes in fine when you type it at that prompt. Also double check you're looking at the right network name. The printer only uses the 2.4GHz band, so if your router puts out two seprate names, one for each band, you need to pick the 2.4 one. If your router uses a single merged name for both bands then it'll just work, but the split setup is a really common reason a printer sees your neighbors fine and not yours.

On the restart question, it doesn't matter much. Power cycling is fine, restarting through the menu is fine, either one brings the network back up and it should grab your WiFi automatically. Then check the new IP in the printer's settings like you would with the cable plugged in. One more thing before you do any of that: poke around the settings menu on the touchscreen itself. Depending on the firmware version yours shipped with, there's a network option in there and you can just pick your WiFi and type the password straight on the screen. No SSH needed at all. Worth two minutes to check before you go the terminal route.

Seconding the nmtui route, that's what finally worked for me too. One thing that tripped me up after the switch: my printer had a different IP on wifi than it did on the cable, and I kept typing the old one into my browser wondering why nothing loaded. So check the IP in the settings again once it's on wifi. Also on mine the touchscreen wifi option was greyed out until I did a firmware update, so if that menu is missing for anyone, updating first fixes it.

One more thing to check if it still won't join after doing the nmtui stuff, some routers have MAC filtering turned on and will silently reject anything new. Mine did that and it drove me nuts for a day because the printer acted like the password was wrong when it really just wasn't allowed on. Log into the router, either turn filtering off or add the printer's MAC address from the settings page, then it connected first try.

One thing that bit me on mine: if your router has a guest network option, don't put the printer on that. I tried it because it seemed tidy, and it turned out my router isolates guest devices from each other and the main network, so the printer connected fine but I coud never reach it from my laptop. Fluke on my part but took forever to figure out. It has to sit on the main network like the cable setup was. Also plug the printer in close-ish to the router first time around, mine was way across the house and the signal was barely enough to boot the connection before it dropped.

One thing I ran into on mine that fits the same symptoms, my router was set to WPA3 only and the printer woud just refuse the handshake, showing the wrong password error that had nothing to do with the actual password. Switching the router to the mixed WPA2 and WPA3 mode fixed it instantly. Also check that the network name isn't set to hidden in the router settings, beacuse a hidden SSID needs to be typed in manually and that tends to fail through the touchscreen keyboard where you can't see trailing spaces. The exclamation mark was never my problem either, mine had a hash symbol and went through fine once the security mode was sorted.

Update: I decided to skip the manual config file and follow the suggestion about the network manager. It was way easier than messing with the text file. After running the tool, the printer is now online and successfully connected to the signal.