Hi,

Our LTE ISP has their own LTE network and they are enabling roaming on a LTE network of another ISP at the same time (this is not roaming in a different country - just roaming on a network of another ISP inside of the same country). I have a situation where their own LTE network has bad connection and it's switching between their own network and the one of another ISP. The other network has a better quality connection.

Is there a way I can lock the modem to the network of that other ISP only?

What do you mean

another ISP at the same time

? Another Default GateWay?
Many functions you are talking are configured in the properties of the modem.
For example, for LTE-R11 you can set fixed ranges (bands) or mode - only GSM, or 3G, or LTE or all at once. These settings are available directly from RouterOS.
If you decide to work with modems of other companies (for example, QuecTell EP06), you will have to study and use the command line from the / system serial-terminal usbXXX chanel = xxx terminal. And of course, copy the AT command system for the corresponding modem

OP is taking about locking on specific PLMN. Similar to roaming abroad when (possibly) all networks are allowed but one is for some reason preferred over others and modem has to be forced to register to that particular network.

To address this issue I'm using the "cell lock" functionality on my Mikrotik r11e-LTE.
Refer to "Using Cell Lock" in the Mikrotik LTE wiki :
https://wiki.mikrotik.com/wiki/Manual:Interface/LTE

I'm using only the physical cell ID that seems to be enough to get a cell lock:
1. get the physical cell id (phy-cellid) from the r11e-LTE:
/interface lte info 0

2. configure the cell lock on phy-cellid, LTE mode only:
/interface lte at-chat lte1 input="AT*Cell=2,3,,,<phy-cellid>"

Explanation of the AT command (from Mikrotik wiki):
AT*Cell=<mode>,<NetworkMode>,<band>,<EARFCN>,<PCI>
where
<mode> :
0 – Cell/Frequency disabled
1 – Frequency lock enabled
2 – Cell lock enabled

<NetworkMode>
0 – GSM
1 – UMTS_TD
2 – UMTS_WB
3 – LTE

<band>
add if band lock is also required (usually can be left blank)

<EARFCN>
earfcn from lte info

<PCI>
phy-cellid from lte info

Comment:
I tried to use the <earfcn> code as well with less success. In my case it seems the cell needs to change the band during the initiation phase.

Sorry for Necro'ing this one. But did anyone find a solution? Is this inherent of the US version of the modem that the cell locking feature is not available?