Yes - in your case, it's not for want of strong signal. Optus must be undertaking major upgrades to their cellular networks in Brisbane, my local cell tower at Carina had upgrades applied last month for a two week period and yesterday I received another SMS and email notification regarding another two week period upgrade which is due mid this month at the same tower.
When I had low speeds on B3(1800) and B28(700) , the B525s-65a showed that Optus allocated Bandwidth set at 10Mhz for DL and 10 Mhz UL. I don't know if Optus use 5Mhz when congestion sets in but I did not see any in my testing results with that bandwidth. Higher speeds on other Bands used 15 Mhz and 20 Mhz for DL/UL and when Channel Aggregation was deployed by Optus resulting in 50-80 Mbps speeds , 20+20 Mhz (cap on the B525 as it is CAT6 rated). Your B818 is in a different class - it can accommodate a much higher level of Channel Aggregation due to its CAT19 rating, 4x4MIMO, 256QAM DL and 64QAM UL Max. .
I have used this site as a contribution to understanding the relationship between the factors that influence speed outcomes - Throughput Calculator from Cellmapper.net - it is an indicative guide as to why speeds will vary but does not include signal quality inputs - these seem to be reflected in the custom variables you input and are theoretical only - there is no distance from cell input.
At the end of the day, the dynamic environment is controlled by the Carrier. Initially when I had sub 5 Mbps DL speeds e.g. 1.22 DL /3.70 UL Mbps speeds, I used the YesCrowd App for Instant Messaging to an agent who did something to my account and asked me to change the APN which at time was not published on the Optus website. My RSRP/RSRQ figures are no where near the quality of yours however speeds marginally improved after that but it was not until I discovered I had the modem placed on the wrong side of the house that speeds greatly improved. The APN is 'CONNECTLTE' in Caps.
Well the perpetual Optus Tower problem is back for Meadowbrook QLD, previously since early March with the stated tower "Upgrade" which was quickly followed by Fault 139980 and now another fault in my area registered registered as fault 150455 and I'm still with 4G.
I registered for the 2nd time, lets see how long this one takes to resolve.