New SSD not showing up in recovery mode

Hello All,

Yesterday I replaced my mid 2015 macbook pro's battery and SSD (upgrading it from 500 gb to 1 tb). The SSD kit I purchased (OWC) came with a little box to put the old SSD in, making it an external hard drive. When I have the old hard drive plugged in via USB I can boot up normally, everything acts normal, I can see the new SSD in disk utility, but when I boot in recovery mode to recover from a time machine backup (which I have stored on a separate external hard drive) the new internal SSD doesn't show up.

I turn on the computer with the power button, hold down Cmd + R, get the 2D globe as it starts internet recovery, once the window pops up with recovery options I can select Disk utility, recover from time machine backup, or reinstall mac OS, all of them show the external hard drive (if plugged in) but not the new internal SSD, even if I wait for over 30 min.

I've tried erasing and reformatting the new SSD, it is formatted to APFS and mounted. I also booted it in diagnostic mode (hold D while powering on) it found no issues.

The latest OS version installed was Catalina, when I try to reinstall OS from recovery mode it defaults to El Capitan.

Any other suggestions would be much appreciated,

Thanks