How do I check or verify a pgp/gpg signature using a .asc PGP signature file? (Can't check signature: No public key)

I'm installing the Ultimaker Cura 3D printer slicer program from here (https://github.com/Ultimaker/Cura/releases/tag/5.1.0) onto Linux Ubuntu 20.04. I downloaded these 2 files:

Ultimaker-Cura-5.1.0-linux-modern.AppImage
Ultimaker-Cura-5.1.0-linux-modern.AppImage.asc

Opening the .asc file in a text editor shows it contains:

-----BEGIN PGP SIGNATURE-----

iQEzBAABCgAdFiEEGInq7S25O/ff+zymwaG5EGnEr1kFAmLWpkEACgkQwaG5EGnE
r1necwgAwO8fqUtXicpJPiIXeFR6L3a2cTc/hLgTgk4Bw8Ey5LKiQyeIsDd3r/vZ
tGiMsb4TrG8WuGIvidBoubuamnIdy2zKyy8Gk1e+MiIgfIWdWIl7KuX/K3GY0oyV
H5rfQWv/g4hCHsDXRpElva79p6W6DYvgdSGeNTpjaeGmLT29OcXCP4wPvSN4izsi
9AU+0DOdq204ZeiGKboXpdPdkWXeyuMJHFdvTlOZVZSb0Ib0zZugSmWYLo8fvK2p
8mrqPMdLu7BMS9ZS/wGrxRfVyOwxk72xuPjGXsrcPXWHtAF5OjvzvCPUzGfnDN10
fVF3+MKS79PQOEYXwAi2hixPCReWNA==
=12yS
-----END PGP SIGNATURE-----

How do I use this .asc signature file to check the main file? I read this page, and the last example seems to apply: https://www.gnupg.org/gph/en/manual/x135.html So I tried this:

gpg --verify Ultimaker-Cura-5.1.0-linux-modern.AppImage.asc Ultimaker-Cura-5.1.0-linux-modern.AppImage

...and I got the following error, as shown in my run output:

~/Downloads/Install_Files/Cura$ gpg --verify Ultimaker-Cura-5.1.0-linux-modern.AppImage.asc Ultimaker-Cura-5.1.0-linux-modern.AppImage
gpg: Signature made Tue 19 Jul 2022 05:40:33 AM MST
gpg:                using RSA key 1889EAED2DB93BF7DFFB3CA6C1A1B91069C4AF59
gpg: Can't check signature: No public key

I tried following the solution in [this answer](https://stackoverflow.com/a/55088831/4561887) , using the RSA key hash printed in the previous output above, and it doesn't work either:

$ gpg --receive-keys 1889EAED2DB93BF7DFFB3CA6C1A1B91069C4AF59
gpg: keyserver receive failed: Server indicated a failure

I'm looking around: [Google search for "ultimaker public key"](https://www.google.com/search?q=ultimaker+public+key&oq=ultimaker+public+key&aqs=chrome..69i57j69i60.8488j0j7&sourceid=chrome&ie=UTF-8) ## Related 1. My question: [Which AppImage should I install (.AppImage vs modern.AppImage)?](https://unix.stackexchange.com/q/711975/114401)