Keep your current FW. No really need to update.
What ISP Service do you have? Cable or DSL?
What ISP Modem do you have? Stand Alone or built in router?
What ISP Modem make and model do you have?
Check the cabling to ensure good working condition, swap cables.
Set up the router using the Basic/Internet/Manual settings
Ensure DNS IP addresses are being filled in under Basic/Internet/Manual?
Turn off Advanced DNS Services if you have this option.
Turn on DNS Relay under Setup/Networking.
Setup DHCP reserved IP addresses for all devices on the router under Basic/networking.
Ensure devices are set to auto obtain an IP address.
Set Firewall settings to Endpoint Independent for TCP and UDP.
Once you have the router going for internet,
The set WiFi mode to single mode N.
Need to find out if you have to use 2.4Ghz or 5Ghz, not all devices support 5Ghz.
Set security to Preferred is WPA2 and Cipher is auto or AES only. Play with this.