Dumb comment 1: This is an OS/3 subforum, which is why you are getting responses related to OS/3 and not OS/5.
(I have not seen this issue in OS/5)
Dumb comment 2: When you say the drives are all in the same workgroup. . .is that the same workgroup as the computer? (I once had that problem. . .the NAS units all wound up with a different workgroup than my PCs)
Less dumb comment:
One factor may be the dual NAT configuration.
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Theoretically, with the PC’s and NAS on the SAME router; they should all be assigned IP’s in the same subnet. . .If they are NOT (i.e. check this!), that could explain much.
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How is the ISP router connected to the NAS network router. . .to WAN port or just another ethernet port on the router.
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Is DHCP in the ISP router turned off?
Just a note: you are gambling a bit with DHCP and mapped drives. I would (and do!) assign static IP’s from the router (not the NAS).
As for master browser option. . .not thinking that. BUT to rule it out. . reboot everything. PC’s first. Old NAS last.