To fix any trust chain issues that you might encounter (especially on mobile browsers!) use: certificatechain.io.
IF for some reason the certificatechain.io does not work for you (happened to me recently) you can get the chain part online from https://whatsmychaincert.com/ create a new file and paste your certificate as the first one and paste the chain right after the original (no spaces and remember to leave the "--- begin..." and "--- end" parts).
Update 21.01.2020
For some certificates from nazwa.pl we had an issue in generating a certificate chain on the above-mentioned sites:Cannot generate a chain because this certificate is self-signed or signed by a certificate authority that is not widely trusted by clients. Details:* The chain ends with a root certificate that's not widely trusted by clients: C=PL, O=Unizeto Technologies S.A., OU=Certum Certification Authority, CN=Certum Trusted Network CA
Using this site helped in certificate chain generation:
https://tools.keycdn.com/certificate-chain
