Hello.
I have a server with centos 7 installed.
I gave the server two disks from dell storege (fiber channel)
I saw my disks with the multipath -ll command, then I mounted these disks.
Then I umounted these disks from this server and mounted them on another server.
Then when I wanted to mount them on this server again, I could not see the disks with the multipath -ll command. I did the same operations on Dell storage.
I don't think that you've given enough information for us to help you significantly.
Multipath is meant to provide multiple routes from a host to the same storage. It's not the same as a multi-server (clustered) environment. Also, you don't say whether the disks that you configured were whole physical disks or a LUN sliced from a RAID array or, for that matter, what exact model of storage controller it is apart from it's a Dell and it's fibre channel connected.
It's quite possible that when you mounted the disks on the second server that a 'reserve' flag was placed on the drives ensuring that no other hosts could access concurrently. If the disks were not properly dismounted from that system then those reserve flags could still be set. A physical disk drive is a highly programmable device and stores such information in its 'mode pages'. If this has happened then you would need a system or bench setup that can 'set all mode pages to default' thereby resetting the reserve flag. If these disks are still configured on the storage controller (again, what is it exactly??) but they won't communicate with anything, then I'd say it's likely that a reserve flag is set. It would depend on what storage/cluster software is installed on that second system as to whether that O/S would reserve disks to prevent simultaneous concurrent access. Two systems simultaneously accessing a filesystem is a recipe for instant filesystem corruption.
I'm just guessing at your problem because of lack of detail. More detail please.