Yes you will need "2 item menus". One on list of all ads, One of type "adsmanager details" (no need to have this menu item visible on the from, could be in a menu that is not displayed on the front). It's not needed to select a specific adsmanager ads for adsmanage details menu item.

So you can decide to display modules depending on the menu items

If you want to use SH404SEF you need to know exactly how Joomla! works with URL.
and what is doing sh404SEF.

Each times a SEF URL is generated, it's generated for ever. For example if the URL contains the name of an ad or article, if you change the article title, this will have to impact on the URL because the first time the SEF is generated the result is stored in database.

Here i think the problem is the same. You have generated some SEF URLS with a specific Itemid, but sh404SEF to prevent duplicated content, will see that in the database there is already a SEF Urls with the same parameters (adsmanager, ad id) but different Itemid. So for sh404 it's the same page so he will use only the first URL generated the other will be marked as "duplicated". So you have to "purge your old URL" if you change some Itemids"

Hello,

To solve this issue, we change the value of the parameter 'Users per page' of Community builder to 20 (the same value as AdsManager).
Then, we searched the links that did not work correctly, for you, it was 'Community'.

After that, the list displayed 2 links, we selected each of them and clicked on 'delete with dupl.'
And that's all .

Best regards.