I advise you to operate with a direct network cable from your PC to the router ...
so the easy solution is to leave lan3 on untag on vlan1 (br-lan)
connect your PC to lan3 and so you can change all the other options without losing access to your Openwrt router
ps obviously you will leave lan3 unchanged (whit luci and ssh access)
lan3
vlan1 = untag
vlan2 = off
vlan 3 = off
vlan 4 = off
vlan 5 = off
if you make changes to the vlans and apply the changes the system will return the state to the previous configuration after 90 seconds (if errors connections)
Another option is to use a USB Ethernet adapter to make all the changes you want without losing access to the router