Thanks Justin, I'll document my tweaks here for anyone else with this scenario.
Secondary modem SIM is Three UK. Upgrade to latest firmware dropped peak performance from 55mbps to about 12mbps.
According to https://3g.co.uk/guides/what-frequencies-does-three-network-use I can only set B1, B3 and B20
My location has stronger reception for B20 than B3, but B20 gives 25% performance of B3.
So by setting the modem to Manual band selection of JUST B3, we are internetting again at reasonable speeds.
NB: I don't know what I'm doing, so comments around these settings are welcome :)