@Clive_A  I have two, stacked, Dell 5548P switches between the ER7206 and the network clients.  It is an 802.1Q configuration.  I'm going to speculate that Cisco built the Dell units, as the CLI commands are near identical, if not identical.

The Dell 5548P is configured for DHCP relay and the ER7206 is confiured to have a DHCP server for each VLAN.

When I define the primary DNS server, for VLAN 300, VLAN members cannot ping each other or the router gateway.  If I do not define the primary DNS server and leave it blank, then the router gateway can be pinged by members, but not, of course,between members.

VLAN 100, which is the factory, un-deletable, VLAN functions OK.  VLAN 100 and 300 are configured exactly, short of IP addresses, the same, but yet the ping issues only impact VLAN 300.

  @martincom PVID is 1 so depending on the dell switches that are like cisco, to get switchport trunk allowed vlan to work without a native the PVID would have to match the tagged number as well, otherwise it won't match the other end.

pvid 100 & tag 100 =

switchport trunk allowed vlan 100

switchport mode trunk

Its possible that the ping is matching 1 and not 300

Thats my experience with it anyway.

  @soupastars If the connection to the switch (VLAN 300) was from an untagged interface that would be correct.  However, that is not the case here.  The interface is a trunk, which passes the tags from the switch to the router.  A trunk will not pass any untagged traffic, except for VLAN 1, which is typically the native VLAN.  For the router to respond to VLAN 1, the receiving port must have its PVID set as 1.

Regardless, this doesn't have anything to do with the issue at hand.

  @martincom SOLVED The issue was a conflict between hardware and software VLAN assignments.

This is a Server 2019 host with a Server 2019 VM.  They both were served by a common, 10gb, NIC.  The host vlan was set in the NIC hardware (Adapter properties\advanced drop down.)  The VM vlan assignment was made within the hyper manager virtual switch tool, where the VM virtual interface is created.  Apparently, it does not like to function properly on top of the hardware assigned vlan.  It had me fooled, as the host operation was 100% and the VM could access the internet, and the LAN briefly, at times..

To resolve the issue, I configured a virtual NIC team on the host.  See the link within this title: Configuring Multiple VLAN Interfaces on Windows  (I would have included an embedded link, but the forum software stated it was "illegal".)  I configured a virtual interface on the team, for the host, and assigned the vlan within.  I utilized the virtual switch, within hyper manager, to create the VM virtual interface, as you would normally, and assiged the VM VLAN within.