GS105 v5 Losing internet when connecting to multiple routers

> I have a Nokia fiber modem [...]

   An actual model number would be helpful.

> [...] connected to my GS105 switch. [...]

   Don't do that.  Most likely, your ISP connection gives you one
external/public IP address, so it must go directly to a NAT router
(WAN/Internet port).  On the LAN side of that router, you _can_ connect
multiple devices (including that GS105 switch).

> [...] One port goes to my Netgear 7500 v2 router. Second port goes to
> Sagemcom router.

   Is your "my Netgear 7500 v2 router" actually an R7500v2, or something
else?  What, exactly, is your "Sagemcom router"?

   Cascading multiple routers can cause multiple problems.  What purpose
does your "Sagemcom router" serve?  If it's used to extend wireless
coverage (and you want to use the R7500v2(?) as your main/only router),
then you may want to configure it as a wireless access point, rathar
than as a full-function router.