The RPi 3B+ has the wifi/bt module from the RPi Zero W, which is apparently "better" than its predecessors as far as hardware goes. Still, the RPi's antenna is simply small comparted to external antennas. In terms of software/drivers there is not much you can do, working with LibreELEC.
Windows laptops & Android have different hardware and different drivers. The one thing you could do is do a speed test with all hardware involved, and see what is the outcome.
Wifi surely can have its advantages, but the outright winner with network speeds is always the device with the cabled ethernet connection.
I've been thru the same issue. My solution was a cheap $10 USB 2.5/5Ghz wifi adapter with a decent antenna (Something like This) and I haven't had any problems. Set to 5Ghz I can stream from my NAS upstairs to my LE downstairs without issues.
Home plug is another solution but a real pain trying to find a "clean" socket.
LE has drivers for most (not all, but most) common WIFI chipsets. Look for something with an Atheros chipset that uses the ath9k driver, it's one of the better written kernel drivers. I'd personally avoid anything with a Realtek chipset inside, but that's just me 
How did you use that? Drivers? Plug and play?
The ones I used were "almost" plug and play. Because they were cheap, they didn't seem to have a MAC address, this was solved Here
Yes I disabled the internal RPi3 wifi with dtoverlay=pi3-disable-wifi in config.txt. I believe you can use both as the internal will show up wlan0 and the external as wlan1.
See previous #6 this was solved Here".
If it's the same error as I had it's due to the lack of a MAC address, so it will connect once with a random MAC address but after a reboot, you'll get a new MAC address, so it will forget everything like password or IP address if static.
Obviously set the MAC address to something notable, especially if you're using, or plan to use more than one adapter. I changed the last 3 digits to the 192.168.0.nnn IP address as I have static IPs. Of course you could always add a DHCP reservation in the router if you want.
Otherwise if not the error, post debug logs. dmesg and kodi.log How to provide log files
