Your WPA settings are in the router settings, under Advanced Settings on the left, then Wireless, then the General tab, then in Authentication Method. At least I am pretty sure they are, as I have a similar router AC66U, and mine are there. Though, I couldn’t get it to connect no matter what options I chose.
So, what I did was enable the Guest network, but with MAC address filtering enabled so that only my Brax3 could connect to it and any other device that tries is blocked.
In the router on the left I chose Guest Network under General, then next to the 5 GHz network (rather than the 2.4 GHz, that is slower) I tapped on the first Enable button, then chose Yes to Hide SSID, gave it a unique name in Network Name (SSID) field, (one I could easily remember for when I type it into my phone when trying to add the hidden Wi-Fi to connect to, like “happy_kitty” 
), then chose Open System in Authentication Method, made sure to leave Access Intranet as Disable (that protects your home network setup in the router), had Router only in Guest network on AiMesh, and first I left the Enable MAC Filter as Disable, then rebooted the router. I left MAC Filter Disable first so I could connect to the hidden Wi-Fi with my Brax3, then be able to select the phone MAC address that shows up once I was connected to add it to the Allow list, to allow only that MAC address to connect, and all others are blocked.
Now connect to that “happy_kitty” hidden network in your phone after router reboot. Go to Network & Internet, tap Internet, scroll down and tap + Add network, in the SSID field put happy_kitty (you could use the qr-code too if you want), for security leave it as none, in hidden network chose yes, in privacy chose used device mac address, and tap save. It should connect and you should hopefully have internet to your phone now. Test that.
Now you want to set the Brax3 mac address in the Allow list of your guest network. So,back in the router settings under General on the left then tap guest network, tap on the details where you should see “happy_kitty” or whatever you used, then in Enable MAC Filter chose Accept, then under Client Name (MAC Address) on the right of that drop down tap the red down triangle, you should see your Brax3 there, probably named android-dhcp-15 (whatever your device name is here), tap on it in that drop down list, it will populate with the Brax3 device mac address, tap the + right of that under Add / Delete, then tap the Apply button. You may need to reboot the router again here after it refreshes, I did. Then reconnect once it all comes back up.
You could try connecting another device to the same hidden wifi ssid to test and see that you have your router set to only allow your Brax3 and no other devices.
It should be known though, hiding an SSID and MAC filtering aren’t effective against hackers, but this is on a guest network, so maybe the worst that can happen is they use your internet connection to slow down the other devices, though I am no expert MAC Address Filtering and Hiding SSID Won’t Protect Your Wi-Fi Network
Very sorry I left out the adapter needed to be able to use with phone with case on. The Acer adapter by itself will not plug into phone correctly without the added USB-A (female) to C (male) adapter. We used the UGREEN 4 pack 40702P4A - Again, sorry for omitting this. A better choice is what ‘BraxFan’ shows us above. That way you can avoid having to use yet another adapter.