C6300 (used as router) - can't access routerlogin; patchy wifi and ethernet connection with Smart TV

Details:

• Netgear device: C6300 used as router only (No cable TV; Internet only)
• ISP: Wave G
• Internet source to router: Ethernet cable from wall to Router Socket 1
• Problematic connected device: Samsung Smart TV UN50JU6500 (up-to-date firmware)

Problem:

• Since first use, have been unable to access routerlogin on devices connected via wifi or ethernet.  Error message: "Not connected to your router's wifi network" (Factually untrue as I was accessing the Internet via wifi on the same laptop)
• Samsung Smart TV now has very patchy connectivity both via Wifi and Ethernet (from Router Socket 2 to TV). Was working fine for 3-4 months until it started having issues.

Attempted fixes:

• Reset router: Sometimes fixes the TV connectivty issue but not every time
• Followed without success: https://kb.netgear.com/app/answers/detail/a_id/27199?cid=wmt_netgear_organic 
• Also followed Samsung community suggestions to manually set default DNS server (8.8.8.8) rather than automatically detect. No fix.
• Can bypass router completely to solve the TV issue (ethernet cable from wall direct to TV) but this is unsatisfactory as I would not have a Wifi network for other devices

Additional notes:

• Verified performance of ethernet cables
• Verified wall socket (no performance issues when plugging ethernet cable from wall directly to TV)
• Verified Wifi and Ethernet performance of router generally (laptops and other devices can connect fine but TV has issues; no access to routerlogin on any device)

Help is appreciated! Super frustrating.