moved to new host — MS doesnt work /error establishing / redirect loop

we moved the site to a new host and new domain name. The urls are updated properly in wp-config.php and there are no plugins. we have multiple sites hosted. we copied the wp files and restored database. siteurl, homeurl are updated to https://new-domain-name, but we are yet to do search/replace in wordpress database (using interconnectit script, as suggested in forums)

we are facing the following cases:

1.

WP_ALLOW_MULTISITE = true/false;
MULTISITE = false;
SUBDOMAIN_INSTALL = true;

or

WP_ALLOW_MULTISITE = true;
MULTISITE = false;
SUBDOMAIN_INSTALL = false;

-> login page, admin page works but cant see “My Sites”

2.

WP_ALLOW_MULTISITE = true/false;
MULTISITE = true;
SUBDOMAIN_INSTALL = true;

-> https://new-domain-name/wp-signup.php?new=new-domain-name
-> ERR_TOO_MANY_REDIRECTS

3.

WP_ALLOW_MULTISITE = true;
MULTISITE = true;
SUBDOMAIN_INSTALL = false;

-> Error establishing database connection

we should be able to access all “My Sites”. Once the network is properly setup in new host, we could proceed to search/replace the urls in database.
Tried looking into the code and searched the forum, but couldnt figure out the solution.

Looking for urgent help. Thanks in advance !!