The thing that has work for me for many years, is to assign static addresses for the Sonos speakers.

However as Sono devices do not themselves support “static ip addresses” you still need a DHCP server to deliver the fixed ip address, and that seems to be a challenge for the OP.

 I have done the IP Address reservation settings in my ISP gateway,  but those reserved IP Addresses keep reverting back to DHCP.  The ISP states this is how it goes.  Their logic is flawed.

 Can anyone recommend a router that will hold those IP Address reservations and works well with Sonos?

MoPac as controlav says Sonos can only use DHCP to obtain an IP address so when reserving IP address’s on your DHCP server (most likely your Router in a home environment) you have to reserve the IP addresses within the DHCP scope.

Domestic Routers are “jack of all trades” and don’t always get everything right and some not even the basics. Some will let you set a static IP for a device but the IP can be from the DHCP range (scope). So your static IP is already in use by a device and the Router goes and gives the same IP to another device which request one. Routers also mix the static / reserved terminology which leads to great confusion for the end user.

 My ISP is Xfinity.  Years ago, with a different Xfinity gateway, when I reserved an IP Address it would stick.  When we moved and I ended up with a new Xfinity gateway.  Now when reserving the IP Addresses they don’t stick.  I check them on occasion and sometimes find they have been reverted back to DHCP.  When I reserve an IP Address I simply use the one already assigned to the device.  Should I be outside the IP range? 

 The only device this has effected so far is my Bryston BDP-2 which, when the IP Address is changed by DHCP the BDP cannot be found and must have the IP changed back to the reserved one or the search path changed to the address DHCP changed it to.  Never could get the Bryston name to work in the path.

 So far Sonos works fine even with DHCP, but would prefer to have those IP Addresses reserved.  Xfinity hints this ain’t possible.

A reserved IP should always be in the DHCP scope, sorry in the UK so unfamiliar with the kit you are talking about.

The Bryson network name / IP address issue is separate, again a domestic router trying to do too many things.  Have a look to see if there is any mention of multicast or mDNS in the Router as this needs to be turned on, mDNS is like DNS but is resolves local IP addresses to host names if you don’t have a DNS server on your local network which you probably won’t have. Another setting is IGMP Snooping or IGM proxy, if you have such setting you can try turning it on to see if that helps alongside mDNS.

You can download a copy of pfSense for free and run it on any PC that has two network ports. My Sonos is very happy on it with reserved IP addresses.

https://www.pfsense.org/download/

If you like it they have hardware from under $200 that has a very good reputation.

https://shop.netgate.com/products/1100-pfsense

We use static DHCP aka DHCP reservations a lot. This time however, my Sonos devices do get a DHCP offer and instead of sending a DHCP request, the Sonos device ignores it and starts over with a DHCP discover.

NOTA BENE: The Sonos device does not know whether it is getting a static address or one from the DHCP pool and guess what? in my case neither static nor dynamic DHCP assignments work. 

Is the DHCP server the same in both set ups and nothing changed in its settings?

Did I understand correctly that ISP modem + TZ300 works. But ISP modem + TZ670 does not? And just using ISP modem also does not work?

I guess the network review matrix (if they are using SonosNet) and other info won’t reveal anything helpful if you access a device that has an ip address with browser:

sonos_ip:1400/support/review

I suspect you have searched and read older threads about similar issue in case those help, e.g. the last message on this thread: