Keeping one IP
When the address has to hold still, and the port that makes it.
The port decides
Port 823 hands out a new address on every request. That suits data collection and breaks anything with a session: a cart, a login, a multi-step form.
Ports 10000 to 20000 behave differently: each port holds its own address for as long as the session lives. Need five parallel threads on five addresses? Take five consecutive ports.
curl
curl -x http://LOGIN:PASSWORD@HOST:10001 https://ipinfo.io/jsonHOST is the gateway address from your access; LOGIN and PASSWORD are your username and password. All three are on the order page and in “My access”.
How long it holds
The address stays while requests keep coming through that port. After a long pause it may change — that is how home networks behave, not a setting.