first, itama natin ang terminology. magkakaiba ang hub, switch at router. wala na dapat gumagamit ng hub sa ngayon. to be fair, i have seen some of those NH20T units labeled "Network Hub".
also, kung 'yung "Coaxial cable" ang ginagamit mo to connect the 2 routers, ay hindi coaxial ito kundi ethernet cable.
ngayon, the 3 devices you mentioned are multirole devices depende sa use case, at s'yempre sa configuration. based on what you described, my best guess would be:
- Technicolor NT20T: mainly performing ONT duties; potentially setup in bridge mode
- Arcadyan AX3600 (AW1000): your gateway/router that does the authentication with your provider
- Arris TG3482ER3: istorbo/panggulo

kung tama ang assumption ko, you cannot simply insert th Arris between the NH20T and the Arcadyan ("Main router ng ISP"). madali lang testingin in ito. ikabit mo directly si Arcadyan sa NH20T in the basement. kung me internet ka (sa basement), ay ito nga ng "proper/main" router mo.
OPTION 1:
kung isa lang ang cable between basement and 3rd floor, pagpalitin mo 'yung dalawa. Arcadyan sa basement directly connected to the NH20T. 'yung cable sa basement end ikabit mo sa LAN port ng Arcadyan, tapos 'yung kabilang dulo sa 3rd floor ikabit mo sa LAN port ng Arris. Setup the WiFi on the Arris (SSID, etc.). not so much an issue kung LAN port to LAN port and connection with Arcadyan, at wala kang iko-connect na wired device sa Arris, but it will be good to ensure you have DHCP disabled.
kaso lang you will have better wifi sa basement (AX) kesa 3rd floor (AC)... na malamang di mo naman mapapansin if you have less than gigabit internet service, and/or wala ka naman local server/NAS na ina-access ng wireless devices. of course depende rin kung marami sa mga devices mo ang AX-capable.
OPTION 2:
from your current setup na NH20T (basement) - Arcadyan (3rd floor)... use another (new) cable to connect the Arris (basement) to the Arcadyan (3rd floor).
OPTION 3:
kalimutan mo na ang lahat ng ito at bumili ng mesh system. TP-Link Deco, etc.
let us know if you get it to work.