@Calvinzxc 

Hi, welcome to the community.

It is suggested to refer to the following link:

The difference between Access Point mode and Router mode on the Deco

Some advanced features are not available in Access Point mode.

But Deco M4 is capable of 1G speed.

Here are some suggestions for your reference:

1. change a new good quality Ethernet cable(cat5e and higher).

2. connect the other port on the first main Deco M4 to a different port on the front router.

3. make sure you are testing the Ethernet speed or 5Ghz speed from the main Deco, not one of the satellite M4 units.

Thank you very much and best regards.