Configuration Issues OpenWRT 19.07.3 and DrayTek Vigor 130

config interface 'wan'                    
        option proto 'pppoe'              
        option username ''
        option password 'pass'
        option ifname 'eth0.7'            
        option force_link '1'                 
        option ipv6 'auto'    
        option delegate '0'

config interface 'wan6'             
        option ifname '@wan'              
        option proto 'dhcpv6'             
        option reqaddress 'try'
        option reqprefix 'auto'

Edit: Once working, give it a day to settle. Your ISP's DSLAM notices the frequent disconnects and doesn't like it, or rather adapts by reducing link speed to counter the 'unstable' connection.