@wixudiwu 

Hi, before answering your questions, we would like to confirm the following details first:

1. Which one is the main router now? MR600 or VR2800? what is the working on both devices? 3G4G Router mode on MR600 and Wireless Router mode on VR2800? 

2. Is the current internet source still ADSL line connected to VR2800? or 4G SIM card inserted into MR600?

3. You mentioned that all wireless clients are connected to VR2800 through a network extender, and some wired clients connected to VR2800 via Ethernet cable, is there any client devices connected to MR600 directly? 

4. The default IP address of both MR600 and VR2800 are both 192.168.1.1, but if they are connected together and both working on wireless router mode, there will be IP conflict, if you didn't change their LAN IP addresses, I guess one of them has already changed IP address automatically to a different subnet, please double check the LAN and WAN IP address on both devices now(If you are only able to access one of them, please let us know the LAN and WAN IP address of that device)