Yes, HB611-SGST is not a good match for 5Gbps plan as it is limited by the 2.5Gbe WAN.
But you can try to utilise more bandwidth if you really want to.
The ONR has one 10Gbe port and three (Nokia XS-240X-A) or four 1G LAN ports (ZTE F8648P).
1) You already connect the main HB611-SGST to the 10G port. That is 2.5Gbps bandwidth.
2) You can connect the second HB611-SGST to one of the 1G LAN ports. That is another 1Gbps bandwidth.
3) You can connect more wired devices to the other 1G ports, that will be another 2Gbps (Nokia) or 3Gbps (ZTE) bandwidth.
In total, you can consume about 4.5Gbps (Nokia) or fully utilise the 5Gbps bandwidth (ZTE).
But if you want, you can also sell the two HB611-SGST and top up a bit to get one Archer BE805 (dual 10G ports, about S$300) to fully utilise the bandwidth.
Most of the people do not really care about fully utilising the bandwidth. And in reality, 99% of the users do not need 5Gbps bandwidth anyway and will be okay with even 1Gbps bandwidth. They upgrade to 3Gbps plan (S$39.90 per month with two HB611-SGST) or 5Gbps plan since the price is similar compared to 1Gbps plan (typically S$40.65 per month with a lousy router).
I can only fully utilise the 5Gbps bandwidth when I carry out OOkla SpeedTest.