OVH configuration is actually quite simple, and is the same for IPv4 and IPv6:
You apply the given IPv4 address with a /32 prefix length, or the given IPv6 address with a /64 prefix length, to the network interface, statically.
You set the default IPv4 and IPv6 gateways to the IPv4/IPv6 addresses that OVH specifies, which are determined in predictable and ...
The "not secure" means the site your bookmark refers to doesn't offer secure connection options. Complain to the site's administrator to fix their configuration. Nothing you can do locally (and right so, just painting over the warning message doesn't fix anything).
From https://en.wikipedia.org/wiki/Maximum_transmission_unit#Applicability -- "with Ethernet, the maximum frame size is 1518 bytes, 18 bytes of which are overhead (header and FCS), resulting in an MTU of 1500 bytes."