You can try to connect 4G router to WAN port only. Check if the problem is related to multiple WAN port. And when you use 3 WAN ports and 4G router, check if your clients can get the corret IP...

Will not make a big difference. But I more likely to use the modem as bridge mode to reduce the NAT devices. May improve the speed a little.

Yes, you can create L2TP VPN. Other people can connect the VPN to access your network or access internet through your VPN. You can refer to the FAQ. https://www.tp-link.com/en/support/faq/2137/

@Salman.amj I think you just need to connect all of your switch in each apartment to Dlink Router. Like the below. Switch B--------Switch A--------(LAN port) Dlink Router Switch C------------| Switch...

@DCi-Corp There should be a tunnel list on ER6020. You can confirm if the VPN tunnel is established according to the list. If you used the same account for your devices, please make sure that the max...

@Ramy When you connect 3 WAN ports, how about the CPU usage and memory? I think you can try to upgrade the latest firmware. It may improve the stability....