@JohnGhost 

You got nothing to worry about.

All these log messages are normal when you're using the modem/router in AP mode - Case 2.

Most of them are because you're not connecting TD-W8980 LAN4/WAN port but using its LAN(1-3) port to connect to your ISP modem with DHCP server disabled.

Same with the time setting - you need to do that manually every time after restart (not so important though), because you're not using TD-W8980 LAN4/WAN port.

If you use the TD-W8980 LAN4/WAN port (in wireless router mode) you'll avoid most of these messages in event logs, but you'll introduce a double NAT in your network.

Bottom line - you're just fine with the current configuration of TD-W8980.

@JohnGhost 

The problem you're describing has nothing to do with your log records.

The issues with 2.4 GHz band may be due to a strong wireless interference around you.

You can use this guide to select the optimal wireless channel for 2.4 GHz band - https://www.metageek.com/products/inssider/

Determine the optimal (less crowded) wireless channel may improve your wireless experience on 2.4GHz band.

Using LAN4/WAN port is irrelevant since double NAT may lead to LAN devices communication problems.

Using WAN port would make sense only if your internet cable goes directly to your TD-W8980 WAN port (i.e. not using an ISP modem/router).