I am connecting from LAN to LAN, which I suppose it should be the repeater mode? (at least I turned off DHCP and any routing features on HG635).
Otherwise I cannot work out why it is only 2.4G working and 5G does not let me to use the network at all.
The first point is that what you are doing is NOT repeater mode, repeater mode is where the device picks up a wireless signal and re-transmits it. This should work as you are connected in the same way as I have and the fact that 2.4ghz works means that at least part of the setup is correct.
The problem may be that 5ghz not setup correctly. When I was having problems with my setup I could not see the 5ghz connection being broadcast from the HG635 on any of my devices because it had exactly the same SSID as my ZyXEL. I only saw the HG635 broadcast when using WiFi-Radar (on Linux) to view everything being broadcast in my area. So I would suggest that you review the settings to make sure you have both 2.4ghz and 5ghz with unique SSIDs and different from the main routers SSID, that way you can be sure which connection you are using.
One other thing is that my HG635 has F/W 1.04T installed as I got TT to downlevel it because of an issue I had while I was using it as my main router so perhaps there could be an issue with your F/W if different. I dont know if you have started from scratch with your setup, in order to get mine working I had to start with a factory reset and then correctly set up all the various settings and it all now works OK. I guess one last question - has 5ghz ever worked on the router when using it normally?
It is difficult to help somewhat not being able to see exactly what you are seeing when testing this but the fact that mine works is hopeful we can sort this.
Stuart