Why this: > https://www.youtube.com/watch?v=1ZJ-pM89N7o > work on ac3, but there is no way to set up on new L900?
Why? Because this Youtube shows something irrational, and completly outdated. (Like using some Windows for Workgroups driver installation procedure in Windows 11, to have SMB file sharing)
In that video the used VLAN has no function. Remove all VLAN things , just use that second bridge, and all will be set with ease
Using a separate bridge per VLAN is the oldest and obsolete method for working with VLANs.
It can still be done. Just as the VLAN can still be set at some (virtual) WLAN interface. Again an old method.
Today VLAN tagging/untagging/filtering is done with the only bridge that might or might not be hardware offloaded to the switch for performance. (Offloading is depending on the switch chip)
Even worse and much older:
This old ROS software had another model for ports on the bridge (with master port), obsolete and outdated, converted to a newer bridge model.
“Note: When upgrading from previous versions (before RouterOS v6.41), the old master-port configuration is automatically converted to the new Bridge Hardware Offloading configuration.”
Either OP understands what he is doing, and adapts, or he is just copying obsolete cookbook procedures in newer devices, what seldom works.
Softwares evolve, methods change, understand what you are doing, and it is an easy case here.
Just add guest WLAN and ethernet ports to a separate bridge. Done! Over, and up to the next setup.
Mikrotik does not suck, but it is a Lego or Mecanno: you have to (learn to) build that little car, train, house, … , and will have much more functionality than dedicated car, train, house toys
Setup your Cisco catalyst switch with IOS 9.2 procedures, and get nowhere operational.
Understand what you do, and last 50+ years as IT engineer. Just copy procedures, and be outdated in no time. (And are not an IT engineer I would hire)
Mikrotik has a steep learning curve, because there are not the Guides, Theory of operation, Reference, and other manuals, like we had with VAX/VMS (1 cubic meter, 32 binders with every release).
But you should not need those manuals or video tutorials for just this very simple Guest wifi problem.
But you can always look for a cookbook procedure, as starting point, don’t copy or follow, just learn and understand … https://www.d4d.lt/how-to-setup-mikrotik-guest-wifi-network