Ethernet network problems under Windows 10

I have a small home-office wired peer-to-peer network comprising:

Dell Precision T5810– main workstation computer: Win10

Dell Precision T1600 – email and internet browsing: Win10

Dell Optiplex GX620 – subsidiary PC for displaying reference files beside the workstation: Win7 Ult

Dell Dimension – PC with legacy data and programmes: Windows XP

Dell Precision M70 laptop – Win7 Pro (not normally connected)

Panasonic Toughbook28 laptop – Windows 2000 (wifi connection)

Dell 1320C laser printer

ZyXEL ES-108A Ethernet 8-port switch

ZyXEL VMG1312-B10A Broadband WiFi router

ZyXEL WRE2205 WIRELESS N300 Range Extender (wifi connection)

My normal working practice is:

I receive emails, with instructions and attachments, from my clients on the T1600

I copy the attachments to the appropriate directory on the T5810

I open the attachments for reference on the Optiplex and work on the project on the T5810

On completion, I reply to the client’s email on the T1600, attaching my completed work from the T5810

However, I am having problems with the Win10 machines not ‘seeing’ the other PCs on the network. The T5810 often disappears from Windows Explorer Network directory tree - both its own and the T1600 show only the Optiplex and T1600, which means I cannot copy the email attachments directly to the T5810. Also, I cannot access legacy files residing on the Dimension or laptops when necessary.

Strangely, when I reply to clients’ emails on the T1600 and wish to attach the completed work files, the T5810 is available in the Thunderbird directory tree.

The pre-Win10 machines can ‘see’ and access all the other devices. The T1600 was originally running Win7 Ultimate and could also see the whole of the network.

How can I restore full access to my network from the Win10 machines?