What I'm using is even better than dumb modem
On modem . You should upgrade TD-W8970 to 22.03.5 for DSA configuration
root@7362sl:~# uci show network
network.loopback=interface
network.loopback.device='lo'
network.loopback.proto='static'
network.loopback.ipaddr='127.0.0.1'
network.loopback.netmask='255.0.0.0'
network.globals=globals
network.globals.ula_prefix='fd38:14e1:71f6::/48'
network.dsl=dsl
network.dsl.annex='b'
network.dsl.tone='av'
network.dsl.firmware='/lib/firmware/vdsl.bin'
network.dsl.xfer_mode='ptm'
network.dsl.line_mode='vdsl'
network.dsl.ds_snr_offset='-30'
network.@device[0]=device
network.@device[0].name='lan1'
network.@device[0].mtu='1508'
network.@device[1]=device
network.@device[1].name='lan2'
network.@device[2]=device
network.@device[2].name='lan3'
network.@device[3]=device
network.@device[3].name='lan4'
network.lan=interface
network.lan.device='br-lan'
network.lan.proto='static'
network.lan.ipaddr='192.168.1.5'
network.lan.netmask='255.255.255.0'
network.lan.ipv6='0'
network.lan.gateway='192.168.1.2'
network.lan.dns='192.168.1.2'
network.@device[4]=device
network.@device[4].name='dsl0'
network.@device[4].mtu='1508'
network.@device[5]=device
network.@device[5].name='dsl0.35'
network.@device[5].type='8021q'
network.@device[5].ifname='dsl0'
network.@device[5].vid='35'
network.@device[6]=device
network.@device[6].name='br-lan'
network.@device[6].type='bridge'
network.@device[6].ipv6='0'
network.@device[6].ports='wlan0' 'lan1' 'lan2' 'lan3' 'lan4'
network.@device[7]=device
network.@device[7].name='br-wan'
network.@device[7].type='bridge'
network.@device[7].mtu='1508'
network.@device[7].ipv6='0'
network.@device[7].ports='dsl0.35' 'lan1.9'
network.@device[7].multicast='0'
network.@device[8]=device
network.@device[8].type='8021q'
network.@device[8].ifname='lan1'
network.@device[8].vid='9'
network.@device[8].name='lan1.9'
network.brwan=interface
network.brwan.proto='none'
network.brwan.device='br-wan'
network.brwan.mtu='1508'
On router
root@wr3000:~# uci show network
network.loopback=interface
network.loopback.device='lo'
network.loopback.proto='static'
network.loopback.ipaddr='127.0.0.1'
network.loopback.netmask='255.0.0.0'
network.globals=globals
network.globals.ula_prefix='fd5b:d2d3:9348::/48'
network.globals.packet_steering='1'
network.@device[0]=device
network.@device[0].name='br-lan'
network.@device[0].type='bridge'
network.@device[0].ports='lan1' 'lan2' 'lan3' 'wan'
network.@device[1]=device
network.@device[1].name='lan1'
network.@device[2]=device
network.@device[2].name='lan2'
network.@device[3]=device
network.@device[3].name='lan3'
network.lan=interface
network.lan.device='br-lan'
network.lan.proto='static'
network.lan.ipaddr='192.168.1.2'
network.lan.netmask='255.255.255.0'
network.lan.ip6assign='60'
network.wan=interface
network.wan.proto='pppoe'
network.wan.username='login@isp'
network.wan.password='password'
network.wan.device='wan.9'
network.wan.ipv6='0'
network.wan.mtu='1500'
network.@device[4]=device
network.@device[4].name='wan.9'
network.@device[4].type='8021q'
network.@device[4].ifname='wan'
network.@device[4].vid='9'
network.@device[4].mtu='1508'
This way Modem ports lan2 lan3 lan4 and WIFI AP are extending my home network to the basement in my case.
Also worth mention is full 1500 MTU on pppoe-wan