Best Practice to switch LAN Interface (LAG)

Hi,

basically I wanted to add a second LAN Uplink from my Sophos XG device to a Linksys LGS308 Switch. I configured a LAG interface on both ends and it seemed to work (LACP was activated), since the XG interface dashboard showed to me that traffic is being counted on both links. However, the I trouble I ran into, was when switching all VLANs (subinterfaces of the LAG interface) from the old LAN2 Port to this new LAG Interface. 

Here is a little drawing of my network. I have 6 VLANs bound to LAN Port 2 on XG. There is a LAG between two LGS switches. My Access Point handles 4 Vlans and has one unique SSID for each VLAN. The Management VLAN for the AP is set to VLAN 2.

After creating the LAG on XG

1. I changed the ip address of the LAN2 VLAN interfaces in order to create new VLAN interfaces for the LAG interface using the old and known ip addresses. I simply count +1 in the subnet

2. I created new VLAN subinterfaces for LAG analogous to the VLANs on LAN2 Port and assigned the former ip addresses of the LAN2 interfaces

3. I put the VLAN interfaces into the correct Zones.

4. I changed the interfaces for each dhcp server from old LAN2 subinterfaces to new LAG subinterfaces

5. I did not change any firewall rules, since the source Zones and the source Networks stay the same.

At that Point I thought that should be it. Two issues occurred:

1. None of my LAN devices could get an ip address assigned by the dhcp server

2. My WLAN Access Point got inactive. The weird part was, that I was still able to connect to one of the four SSIDs and get access to the internet, everything else did not work. Like ping internal devices with fixed ip addresses etc.

What configuration step did I miss? Or is there any best practice for switching from one LAN Port on the XG to another (in my case a LAG interface)?

Best 

Peter