Flint 2 GL-MT6000 - VLAN

I need to create 3 vlans. This need arises from the fact that I have a Synology RT6600AX router with two MR2200AC mesh points attached. Unfortunately, the RT6600AX, when used in AP, the LAN ports are all trunks and belong to the main network, so you must necessarily use a managed switch that I have. On the flint, I created a VLAN device, then activating filter vlan, I created vlans 20, 30 and 40, assigning them to port 5 of the flint. Vlan20, 30 and 40 "U" while on the port I indicated "T" so that it transports (if I have understood correctly) the three vlans through that port to the switch. Then I created the bridge interfaces each with the name of each individual VLAN, for creating the subnets with DHCP. I chose port n.1 of the switch where to connect the bridge between flint and switch and here my enormous doubts begin! On the switch I was able to configure the three VLANs which I then also configured on the three ports where I connected the three synology. I was convinced that I had to configure the three VLANs as "U" but by doing so the VLANs did not receive IPs and consequently did not activate. When indicating "T" instead, the three wifi networks attached to the VLANs with IP assignment to the devices immediately began to work and everything works! What doesn't add up to me, however, is why do I have to set everything to TAGGED? I also understood that VLANs cannot communicate with each other unless you set firewall rules. So why is it that if I connect to one of the three wifis I can ping the other two? Any help would be a godsend and I thank you in advance.

VLAN20 VLAN30 VLAN40 Port5
"U" T
"U" T
"U" T

If you have diagram (draw.io) and screenshots it will help to understand.

First of all thanks for your answer.
The following figures show the configuration.

Here I created a Bridge called VLANS assigning it the physical port of the Lan5 router.


Here is how I set the VLANs:


Then I created the interfaces:


The following shows the configuration of the synology RT6600AX router, which is set as an AP and to which two other MR2200AC routers are connected which, due to the conformation of the environment, takes place via LAN. When the 6600 was the primary router, the MR2200s were always connected via cable directly to the RT6600.


Finally this is the switch configuration. Port 5 of the flint carries the VLANs to port 2 of the switch while the RT660AX, MR2200AC and the second MR220AC are connected respectively in port 3,4 and 5.


I'm not very convinced that I did everything properly. Is there something wrong with this configuration? Is there anything that needs to be changed or set differently?
Thank you

I am not good at this part. But in the router settings you bridged these vlans. Isn't this a problem?

The vlans should not be bridged. But should be in individual zone and forward to WAN.