Step one turn on PC directly connected to ModemsbSB6121 without Wi FI router being involved.
Step two go to Command prompt type in ipconfig (press enter)
Step three note IP address of default gateway and ip address issued to P.C.
Step four type in default gateway address into browser and log into Modem.
Step five look for DHCP Server and allocate IP addresses from at least 50 for example 192.168.0.50 starting ip lease ending ip lease 192.168.0.254
Step six connect Wi Router to P.C. via ethernet cable only - remove ModemSB6121
Step Seven shutdown and restart p.c
Step Eight go to Command prompt type in ipconfig (press enter)
Step Nine note IP address of default gateway and ip address issued to P.C.
Step Ten type in default gateway address into browser and log into Wi Fi Router
Step Eleven Change IP address on Router to 2 as example 192.168.0.2. (it has to match in that modem device would have been 192,168.0.1)
(if it was 10.0.0.1 make wi fi 10.0.0.2 make sure subnet mask is the same like 255.255.255.0)
Step twelve Change DHCP Server to off
Step Thirteen may need to set default gateway to MODEM IP address for example 192.168.0.1
Step Fourteen plug modem router into Wi Fi router may need to plug into WAN port on WI FI router .
Step Fifteen shutdown and Restart Computer
Step Sixteen go to Command prompt type in ipconfig (press enter)
Step Seventeen type in PING 192.168.0.1 (if this is modem IP address and default gateway IP addess)
All this above is to avoid IP address conflicts or mismatching
Subnet mask should be 255.255.255.0
either it uses 192.168.0.1 or it uses 10.0.0.1 make sure Wi FI router is set to different address to Modem Router.
I suspect either a clash occurs or your P.C gets a different IP range from ther WI FI DHCP server.
Happy troubleshooting !