Complicated story but basically it worked fine on a 2955 but not on a 2960.

AIUI, every "ADSL customer" scenario has a customer router which knows nothing initially about its public IP, and gets that IP allocated by the ISP's DHCP server. If the customer is on a fixed IP, the ISP
merely configures its server to always dish out the same IP. Our router (the old or the new) never had any public IP configured.

On the 2955, the config for USB is under the WAN tab, and typically you set up WAN1 for ADSL and WAN2 for the USB backup. And both WANs are set up with Fixed IP UNchecked so enabling the acquisition of the public IP from the ISP.

On the 2960, the USB is a separate tab, unrelated to WAN1 or WAN2, and there is no config there for anything relating to any IP.

The 4G USB stick config on the 2960 has two options: 3G/4G PPP and 4G DHCP. The 1st one picks up a spurious public IP (actually the 1st one on a 2nd subnet we have from the ISP). The 2nd one connects but the "public IP" showing in the router is 192.168.1.100 which doesn't do anything at all, perhaps because our internal LAN is 192.168.1.* but the USB stick (Huawei E3272) has no config for the IP, to change it to say 192.168.2.100.

The internet is full of people tearing their hair out over this and of course no solutions appear.

I have spent many days on this now. First we had to change out the 2955 because we got VOIP phones and they got intermittent packet loss periods lasting for 10 secs. The 2960 has various other issues with VPN operation (it is actually very different from the 2955 in various subtle defaults) which were eventually solved. Now I am trying to fix the USB backup mode (Andrews & Arnold) which worked fine on the 2955 but on the 2960 it gets the wrong public IP and the original dongle (ZTE MF100) is too slow for VOIP, being just 3G (really poor quality). So I got the E3272 which is 4G and that is even more broken. I now have on order a ZTE MF821D in the vain hope that it is like the old MF100 but does 4G... But we will still be stuck with it picking up the wrong public IP from the ISP.

Maybe the Huawei dongle has a config via telnet on which the stupid 192.168.1.100 IP can be changed.

VOIP is a right PITA. We had 20 years of solid operation on ISDN2 on a Siemens Hicom PBX. The Snom SIP phones are pretty crap too. So many button pushes to transfer a call and most call transfers fail anyway so one has to walk over to the phone on which the call was picked up. We went to VOIP to save the BT line rental charges (we have very low call volume).

I cant help with the usb but that 10sec packet loss will have been the DoS defence kicking in for SYN, UDP or ICMP flood probably, fallen for that before many a time.

I did think of that

I got caught by it when setting up auto sync for photos from my phone to the PC, using a samba app on the phone to access the LAN (via wifi).

That whole section in the router was totally disabled.

I did suspect that, with the buggy software in the 2955, maybe some of it remains enabled. But the issue was not consistent. It happened mainly within the first minute of a call, and then sometimes repeated every few mins and sometimes not. The duration was always about 10 secs.

BUT... another 2955, with a slightly different config (the SIP phone on a local IP and the ISP's IP and ports port-forwarded to the LAN IP) there are no breaks. Figure that one out...

Back to my original post, does anyone know what happens when an ISP delivers the public IP to a router? I think it is via DHCP but A&A told me it is via Radius (which I don't know anything about). In any case, the 2955 got the same IPs on ADSL and 3G, whereas the 2960 gets the right IP on ADSL but picks up the first of a subnet on 3G. For info, let's say the public IPs are

200.201.202.203
250.251.202.204/29
250.252.203.205/29

The 2955 would pick up 200.201.202.203 for both ADSL and 3G.
The 2960 picks up 200.201.202.203 on ADSL (correct) but 250.251.202.204 on 3G (wrong).

A&A can't explain it.

I will have another data point when the ZTE 4G dongle arrives, but I am not hopeful because the ZTE 3G one (MF100; worked on the 2955 for ~10 years) does exactly the above, and the Huawei one likewise.

I am wondering if I should consider some ethernet-connected 4G box, and then it could connect to the 2960's WAN2 port, sidestepping its crippled "USB WAN" functionality. Are there boxes which take a SIM card and which are a "pure PPPOE bridge", just like say a Draytek 130 ADSL modem, and which have an ethernet port?