WP24

That was a bug, it’s fixed now in the current version (1.7.1).

Maybe this helps: https://www.wpglobalsupport.com/create-custom-add-cart-urls-woocommerce/
Perhaps you could then use a purchase link in the domain check similar to: https://yourdomain.com/?add-to-cart=42&attribute_domain=[domain].[tld]

I’am not very familiar with WHMCS, but regarding this article your link sould be something like that: https://spiderhoster.com/portail/cart.php?a=add&pid=1&sld=%5Bdomain%5D&tld=.%5Btld%5D

Which WordPress version and which theme are u using?
Could you provide a link to the page where you inserted the domain check?

Unfortunately the whois server is only available for registered domain sellers: “Direct inquiries to our server are only enabled to REGISTER officials of the .EC or of some companies that sell domains that have requested it.” (Original answer: “Las consultas directas a nuestro servidor sólo están habilitadas para REGISTRAR oficiales del .EC o de algunas empresas vendedoras de dominios que así lo han solicitado.”)

But I’am working on a solution to somehow support this TLDs too.

These TLDs are not added to the plugin yet.
The official whois server (whois.nic.ec) is not working, so there is no possibility to check those TLDs.
I asked nic.ec for a solution, if there is one I will add these TLDs as soon as possible.

The option “Invalid input” is now added under texts & colors.

<div id="dc-result-1" class="invalid-field">
  <span style="color: rgb(140, 0, 0);">Please match the requested format</span>
</div>

The option “Dot in drop-down” is now added under advanced settings.

I use a multisite and it works without problems.
What exactly is not working? Installing, settings or using the shortcode on a page?

The current version of the plugin (1.6.0) now has the selection type “Drop-Down Grouped”, where you could add labels (groups) like that: [Group 1], com, net, org, [Group 2], info, eu

I did not find a alternative yet, if I do I’ll add it.

The .es TLDs are only searchable if you are autorized by red.es (“Access will only be enabled for IP addresses authorised by Red.es.”). Therefor they are not supported.

The jQuery of your theme seems to produce an error: https://prnt.sc/vrsfcx
Since the domaincheck plugin uses jQuery too, this error could prevent it from working.

It looks like the short code is used twice on that page.
If that is intended enable the “Multiple use” option in the general settings.

You are using the block editor (Gutenberg)? The block editor has a shortcode block, but the shortcode should also work in a text block.
Could you post a link to your page, so that I could check it myself?