The config you posted is obviously the one with a static IP assigned on the LAN.
In theory, you should be able to make this work as a DHCP client by modifying the file to look like this:
config interface 'lan'
option type 'bridge'
option ifname 'eth0.1'
option proto 'dhcp'
Is this how you had performed the change, previously?
Could you ssh into the router and post the output of the following:
opkg list-installed | grep dhcp
On my router (19.07.1 official stable release build), I see the following:
odhcp6c - 2019-01-11-e199804b-16
odhcpd-ipv6only - 2019-12-16-e53fec89-3
This makes me wonder what package has the DHCPv4 client that runs on the system by default... it's obviously there, but not as a standalone package.