Trying to play Gears of War 4 in online coop mode, but the player invited by the player hosting gets kicked off the servers time and again?

Check the NAT setting in the network settings on the Xbox. The host must have an ‚open‘ NAT – not a ‚closed‘ or ‚moderate‘ one.

In order to fix a wrong NAT setting, head to your router. In your local network check the settings for your Xbox. Enable UPnP and that should fix all NAT troubles and make coop games possible.

Bonus content for fixing network related trouble:

Make sure these URLs are whitleisted in the sinkhole of your choice:

assets1.xboxlive.com
activity.windows.com
arc.msn.com
attestation.xboxlive.com
cert.mgt.xboxlive.com
client-s.gateway.messenger.live.com
clientconfig.passport.net
continuum.dds.microsoft.com
ctldl.windowsupdate.com
def-vef.xboxlive.com
device.auth.xboxlive.com
eds.xboxlive.com
fe3.delivery.dsp.mp.microsoft.com.nsatc.net
help.ui.xboxlive.com
licensing.xboxlive.com
mobile.pipe.aria.microsoft.com
notify.xboxlive.com
profile-prod.svc.gearsofwar.net
settings-win.data.microsoft.com
sls.update.microsoft.com.akadns.net
title.auth.xboxlive.com
title.mgt.xboxlive.com
userstats.xboxlive.com
v10.events.data.microsoft.com
v10.vortex-win.data.microsoft.com
v20.events.data.microsoft.com
v20.vortex-win.data.microsoft.com
watson.telemetry.microsoft.com
web.vortex.data.microsoft.com
www.msftncsi.com
www.xboxlive.com
xbox.ipv6.microsoft.com
xboxexperiencesprod.experimentation.xboxlive.com
xflight.xboxlive.com
xkms.xboxlive.com
xsts.auth.xboxlive.com