Hello, I have purchased multiple SSL certificates from various sites, including GoDaddy and ssls.com, and uploaded them to MikroTik. However, none of the certificates worked correctly. Could you assist me with this issue?
Please elaborate on the “none of the certificates worked correctly”. What are the certificates used for?
There’s more to certs than having a certificate. It must be imported, it must be valid and valid for the use case you have for it. Defining the use case of what you want to do is always a good idea 
Do make sure that you upload the certificate with the private key material. That requires either a p12 file or a “.crt” (public) and typically a “.key” file (private).
The certificate must have a subject name and these days also a subjectalternativename (SAN) filled with some time or even wildcard, if the CA allows it.
router.mydomain.de, *mydomain.de could be an example.
Then your DNS server must resolve such a name fitting to the certificate subject to the IP address that your router actually has. For example, router.mydomain.de to 192.168.88.1.
Another public example: The certificate for this forum is *.mikrotik.com and a few other sites. That this certificate is for a webserver and yours is for a web server on a router is irrelevant, both are using it for the same thing.
Does your domain/subdomain already pointing to your public ip-address?