Hello,
I have RB750. Installed fw 6.41.4 OS - 6.41.4.
I have following connections - 1. Internet link, 2 - VPN -link, 3- tech link, 4 - LAN link, 5- AP-link(at AP I have 2 SSID - 1 for internet, 2nd for LAN).
According this I added IP addresses and 2 VLANs(for AP link). To make AP management I added it to bridge.
So I have following
Internet IP assigned to Eth2
VPN IP assigned to Eth1
LAN IP on bridge interface - br1
VLAN -LAN IP2(this is separate network and has different routing policies)
VLAN - GUEST( This is guest network, internet connection only)
Tech IP assigned to port 4
In br1 I added eth5 and eth3. RSTP is on.
In this configuration I have strange moment. TechIP do not work. I try to ping network dev in this subnetwork and they are not available.
All devices are connected to switch(unmanagement) and then connected to MT.
I take my laptop and connect to this switch. All devices are accessible,freely pingable. I am able to ping MT.
I add this port(eth4) to br1 and MT start ping devs, but! some devices are not available.
This is random device.
I changed switch, but this do not help.
I check arp, unpingable devs are not in table. Table is short about 21 devs.
Suppose this is not limit.
Does anyone know where issue is ? How to fix ?

It's entirely clear how your LAN setup looks like and what's configuration of your MT from what you wrote. At least not to me. It would help, if you could post a (not too) simplified drawing of your LANs and result of command "/export hide-sensitive" (systematically replace any reference to external IP addresses with some place-holders).