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You could use an old router if it has a WiFi bridge mode. Not all routers support WiFi bridge mode. Check the routers settings for WiFi bridge mode or a WDS setting in the WiFi settings.
You could also use a WiFi extender with an Ethernet port such as the TP-Link TL-WA850RE that costs about $39 new.
If the camera requires port forwarding it want work over the mobile network as the modem will not have a public IP address.
There are some business mobile broadband services with a GPTEXB3 Profile have a public IP address. You could try getting a business mobile broadband account with a public IP address.
The other alternative is to use a camera that does not require port forwarding.